While rewatching the 2009 U.S. Open final between Roger Federer and Juan Martin del Potro, what sticks out most is the discomfort among the coaches, family, and friends in Federer’s box. The in-seat shifting, the head-scratching, and the downturned eyes were rooted in the distress of the moment — seeing their man’s chance to win a sixth straight Open title slip away at the hands of a 20-year-old in a sleeveless shirt and neon-yellow headband — but their anxiety seemed to have as much to do with what might lie ahead. Andy Murray and Novak Djokovic had begun to emerge, but were not yet consistently challenging Federer and Rafael Nadal. Murray and Djokovic were also recognizable threats, corollaries to the model of tennis success that Federer and Nadal had established: quick movers with fluid, rangy bodies, solid baseline games, and decent serves, capable of parrying their opponents’ best shots but rarely executing game-changing plays.

Del Potro, however, was a different menace entirely. At 6-foot-6, his victory made him the tallest Grand Slam champion in history, and his game was built not on defense but on the attack. His forehand rocketed around the court at 110 mph and his serve was an emerging weapon. But del Potro was no hulking bruiser. “You just don’t see this on a tennis court, someone so big and so smooth,” John McEnroe said at the time, predicting that del Potro could win multiple Grand Slams. The Argentine finished 2009 as the youngest player in the top 10, and Sports Illustrated predicted that, barring injury, he would dominate the next decade of men’s tennis.

Then came the injury. Early the following year, during an exhibition leading up to the Australian Open, del Potro hurt his wrist. Within the year, his ranking dipped to no. 484, and against the Big Four of Federer, Nadal, Djokovic, and Murray, a group he was supposed to expand to Five, he lost all but two of his next 16 meetings. The current golden era in men’s tennis has been littered with the carcasses of those sent back down the mountain, and it seemed del Potro might be another casualty.

But his success at the Open came so early in his career that there was still time to rise again. This year, at Wimbledon, del Potro reached his first Grand Slam semifinal since 2009, a bruising five-set match against Djokovic that del Potro called the best of his career, even though he lost. He then won his next tournament, in Washington, D.C., besting top-ranked American John Isner in a final that was most notable for del Potro’s return of an Isner approach shot. Del Potro swatted the ball back with a forehand so ferocious that Isner had to leap out of the way to keep from being hit in the groin. Del Potro smiled while Isner removed his hat and scratched his head. Isner told me later that the only time he had seen a forehand hit that hard was when James Blake used to try to intentionally hit him in practice. “I couldn’t do anything about it,” Isner said.

After the trophy ceremony in D.C., del Potro returned to the locker room, where members of his team had gathered with a crate of Coronas. They shook the bottles and proceeded with the customary spraying, but del Potro’s heart seemed only half in the celebration. He had his bottle weaponized, but he paused his attacks occasionally to wipe the suds from his hair. Winning the tournament in D.C. would be a signature achievement for many players, but it seemed as if the mess of a beer-soaked celebration was hardly worth it for del Potro, a Grand Slam champion.

Four years after the biggest moment of his career, del Potro remains the only man outside the Big Four to win a Grand Slam title since early 2005. (Seventeen women have won Slams in the same period.) He has yet to return to a final, but he entered the U.S. Open ranked no. 6 in the world, his highest position since 2010 — higher, even, than Federer, who has slipped to no. 7. If forced to choose someone to join the Big Four, or perhaps replace its most senior member, del Potro remains the best available bet.

Del Potro was seeded seventh in this year’s Western & Southern Open, a tournament in suburban Cincinnati that represents the final major tune-up before the U.S. Open. It was a chance to assess del Potro’s form and see if, as many observers were predicting, he finally was ready to join the ranks of the Big Four. “He’s a better tennis player now than he was in 2009,” Darren Cahill, an ESPN tennis commentator, told me in Cincinnati. “Of all the other guys right now, he’s the one who can get there.”

Del Potro took the court for his first match in what would be his uniform for the week: a bright yellow Nike Dri-Fit shirt and white shorts with a barely perceptible plaid. The shorts fit del Potro’s persona. Whatever flair one might expect from a professional athlete is muted by del Potro’s generally suppressed demeanor. His sunken green eyes seem more suited for a Nicholas Sparks novel than a locker room. His deep, slow voice, even when talking in Spanish, sounds like it could belong to an anthropomorphic cloud on an animated children’s TV show. On the court, he spends most of his time between points staring at the ground.

His first-round opponent was Nikolay Davydenko, whose career can be read as yet another cautionary tale about fleeting success at the top of the men’s game. (And this without even delving into Davydenko’s central role in the longest-running match-fixing inquiry in tennis history, of which he was eventually cleared.) Just two months older than Federer, Davydenko rose as high as no. 3 in the world and remained in the top 10 for almost five years. Yet he was never able to muscle his way to a Grand Slam title, and he never became a serious challenger. His career record against Federer is 2-19.

Rehashing the dominance of the Big Four in men’s tennis over the past decade has become a tiresome statistical game, but suffice it to say that there are several other men who could currently be Grand Slam champions if one or two of the game’s Quadrumvirate had chosen golf. This dominance by so few over such a length of time is singular in the history of men’s tennis, and among the most dominant decades constructed by a small group of athletes in the history of modern sports.

Most of the supposed challengers to the Big Four have been solid players without any extraordinary physical gifts that might make them more than an occasional annoyance. Davydenko had been one such player: At 5-foot-10, he was quick, with one of the most consistent baseline games on the tour, but one capable only of glancing blows, not knockouts. Against del Potro, he not only gave up eight inches of height, but considerable firepower, barely making an effort to reach many of the blasts launched from del Potro’s right arm. The Argentine won in straight sets.

When fully loaded, del Potro’s forehand is the most deadly baseline shot in the game; somehow, it seems to only gain power when he is on the run. In Cincinnati, Djokovic called it the game’s best. Del Potro utilizes something resembling an Eastern grip, a vintage style largely replaced in today’s game by Western grips, which produce considerable spin but require players to whip upward on contact. (Think Rafael Nadal.) Del Potro’s grip and simplified backswing allow him to hit a flatter ball with less spin and more speed. His forehand averages 2,000 revolutions per minute, while Nadal’s typically clocks in somewhere around 80 mph (though he is capable of much more) with 3,200 revolutions. At Wimbledon, against Djokovic, one of del Potro’s forehands had been measured at a mind-boggling 113 mph. When the point ended, Djokovic simply bent over in exhaustion.

Live tennis is filled with moments when the crowd collectively inhales after a particularly well-struck ball is loosed, and del Potro’s forehand is among the tour leaders in gasps per attempt. In Cincinnati, after one point in which Davydenko had to return several del Potro forehands in a row, I watched him grimace and stretch the fingers on his racket hand. It looked like it hurt.

Del Potro’s nickname is the “Tower of Tandil,” a nod to his hometown, a city of 100,000 in the mountains south of Buenos Aires. Like many non-soccer-playing athletes from Spanish-speaking countries, del Potro was a frustrated futbol-er who turned to tennis. He began working with Marcelo Gómez, an instructor at a local club who had trained several other pros. Del Potro is the most successful of the five top-100 tennis players who have come from Tandil. His stock reply to questions about what has made this area of Argentina so prolific athletically is first Gómez and second “the beef.”

By the time he faced Federer at the U.S. Open in 2009, del Potro had already been ranked in the top 10 for nearly a year. He had turned pro at 16, in 2005, and sprinted into the top 20 in 2008 after winning four straight tournaments and 23 straight matches. “Juan Martin is a different-level player,” Andy Roddick said after losing to him during that streak, noting how well del Potro played for his height. Roddick, who stands at 6-foot-2, continued: “I remember I was kind of big once upon a time as far as tennis standards.”

Still, the Open final would be del Potro’s first Grand Slam championship match — and just his second time on the hard court in Arthur Ashe Stadium — so the night before, unable to sleep, he spent four hours on the phone talking to friends from home, “not about tennis at all.” Del Potro is exceptionally close to his family — broadly defined, he estimates that group to include close to 200 people — and a typical post on his Facebook page will show him reading a book to a young relative. (Others of late: kissing Minnie Mouse at Disney World and attending a Bruce Springsteen concert — his favorite song is “The River” — with his coach.) But, while Federer’s box had become one of the most sought-after spots on New York’s post–Labor Day social calendar, del Potro had arrived in New York without an entourage: When Open officials told del Potro he had 15 tickets to give out for the final, he could only come up with three people to take them.

Del Potro was not prepared for the new reality of being a Grand Slam champion. (Asked what he would do with his $1.6 million in prize money, he said that he might buy himself a cheesecake.) Winning the Open vaulted Del Potro to the peak of his country’s sporting hierarchy — that year he beat Lionel Messi, the reigning FIFA Player of the Year, in a vote to name the country’s top athlete — and the increased expectations meant elevated scrutiny when del Potro hurt his wrist in 2010. Del Potro was largely silent about the injury, and as time went on and he was unable to return to the court, the more benign reports said he was simply dealing with tendinitis, while uglier gossip in the Argentine press suggested he was suffering from depression and panic attacks. Was the injury in del Potro’s head or his wrist? The whispers eventually grew so loud that he sent a statement to a pop radio station in Argentina that was vague enough to only fuel the chatter: “As you all understand, this is not a happy moment in my life. But I’m used to fighting adversity and I have all the forces to move forward … I will not explain what doesn’t exist.”

Del Potro had a number of doctors examine his wrist, to no avail, before he was advised to visit Dr. Richard Berger, an orthopedic surgeon at the Mayo Clinic in Rochester, Minnesota. “The problem wasn’t in Juan Martin’s head,” Berger told me recently. “It was in his wrist.” I had reached Berger after one of del Potro’s matches in Cincinnati. Berger had become a fan, and let out a “Woo-hoo!” when I told him that Djokovic, who would have been del Potro’s next opponent, had been knocked out of the tournament. When del Potro arrived in Minnesota, he had trouble identifying exactly where the injury was located, so Berger booked several courts at a nearby tennis center, for privacy, and told del Potro to play as hard as he could until the pain was too much to bear. When it was, Berger applied a local anesthetic, and within several minutes, the pain was gone, and del Potro went back to playing at 100 percent.

That experiment told Berger that the problem was in Del Potro’s extensor carpi ulnaris, a common locus for injuries in “swing sports” — tennis, baseball, hockey. Berger had requested that we speak via Skype so he could use a set of pipe cleaners to show how the tendons in del Potro’s wrist had split, an injury similar to one he had treated for Washington Nationals outfielder Jayson Werth. “This tendon is really at the core of forearm rotation,” Berger said. “Any sport where there’s a hitting impact, and you’re trying to use your arm as this rotating lever to move the club or racket or whatever, you’re in increased risk.”

Del Potro spent the next several months playing very little tennis. For rehab, he would lightly swing a racket without strings before moving to light rallies at the net. (Werth began by swinging a whiffle ball bat and then hit a plastic ball off a tee.) When he returned to the court in fall 2010, his play highlighted how long he had been away. Del Potro lost in the first round of his first two tournaments, without winning a set. His performance improved in 2011, and again in 2012, when he won four smaller tournaments, but his results in Slams remained disappointing. “Missing a year with the wrist injury set him back considerably more than anyone anticipated,” Cahill, the ESPN commentator, said. “Those four top players were stretching their lead from the rest of the pack.” It didn’t help that del Potro seemed to always be suffering new injuries — an abdominal strain, pain in his left hip, pain in his right shoulder, a hyperextended knee, a twisted ankle. Once his right wrist had healed, his left one started to act up. While Federer had managed to play his entire career without serious injury, del Potro’s body seemed like it might ground his career permanently.

During del Potro’s recovery, he received text messages of support from each member of the Big Four. “That was a nice signal,” he told The Independent at the time. “It showed they were still thinking of me. That is more important than the actual game. If the others care about you it means you are a good person. For me that’s more important than playing good tennis.”

One perceived obstacle to del Potro’s rise to the top of the game is that he might be too nice to be a champion. In an era already marked by genial relations between the men at the game’s peak, del Potro has earned a reputation as perhaps the tenderest of all. In his third-round match in Cincinnati, against Feliciano Lopez, I watched him call a double bounce against himself that the referee had not noticed, the equivalent of an outfielder admitting he had trapped a line drive. He crosses himself after every victory — he visited the new Pope, an Argentine, earlier this year, and gave him one of his rackets from the 2009 U.S. Open — and del Potro is almost laughably quick to humble himself next to others. After the Washington, D.C., tournament, Isner told reporters that he would put del Potro “maybe the smallest hair behind guys like Djokovic and Murray … He could very easily right now be the third favorite.” When reporters told del Potro what Isner had said, he seemed bashful. “He said that?” he asked, looking over at an ATP official who nodded confirmation. “Wow.” Of Isner, a fine player with next to zero chance of winning the U.S. Open, he said. “I would say he’s going to be a favorite too.”

Isner was Del Potro’s opponent again in Cincinnati, this time in the semifinals, after the American had managed a stunning upset of Djokovic in the quarters. Listed at 6-foot-10, Isner is one of the few players on the tour taller than del Potro. Except on the serve, where simple physics comes into play, excess height does not offer a clear-cut advantage in tennis. In some departments — movement, reaching balls that skip low off the ground — it can make things more difficult. In fact, many taller players (Isner included) have been accused of understating their official height. “I call Yahtzee on that 6-foot-6,” Brad Gilbert, one of ESPN’s color commentators, said in Cincinnati, of del Potro’s listed height. Gilbert may not understand Yahtzee, but the point was clear. “Del Potro is clearly 6-foot-7,” he went on. “But he doesn’t play like the typical 6-foot-7 guys. I watched him in the juniors. He grew a little later.”

The average height among the men’s top 10 is 6-foot-1, and for whatever variety exists in their games, the Big Four are all relatively similar in size — between 6-foot-1 and 6-foot-4, from 175 to 190 pounds. Their success has been predicated on the fact that though they each have strengths, their games are generally complete from serve to volley. What has held back taller players like Isner is the one-dimensionality of their games: His serve is the most fearsome in all of tennis, and his forehand, when struck well, can be dominant. But he lacks the all-court game necessary to dependably compete with the top players.

Del Potro leans most heavily on his forehand, but what has made him such a tantalizing talent is the combination of size, power, and finesse that suggests he can match the top players skillwise, with the added ability to occasionally overpower them. His two-handed backhand is consistent and solid, and Cahill has noted improvements both in del Potro’s serve and his willingness to attack the net, where his height and length can make him an intimidating presence.

If there is a weakness to del Potro’s game, it’s that he sometimes appears exhausted and lethargic. Despite the deadliness of his running forehand, he would rather not hit it. “I don’t like to run,” he told me, after I relayed Djokovic’s comment that his shot was the best in the game. Against Djokovic at Wimbledon, facing match point, he responded to a lob he could have easily retrieved by simply waving his arms, hoping to encourage the ball to go long. (It did, by three inches.) “For him, it’s double, if not triple the effort to spend as much energy on the court as it is for me or for somebody else,” Djokovic said in Cincinnati. “That’s why physically for him, it’s always a struggling kind of sensation.”

In their previous match, in Washington, del Potro had dropped the first set, then steamrolled Isner the rest of the way, losing just three games in two sets. This match appeared to be headed toward a similar result, without the slow start. Del Potro was returning Isner’s dangerous serve with relative ease and he seemed the fresher player. Del Potro took the first set, then earned a break and two match points on his own serve in the second, a margin that should have been decisive. Then, del Potro double-faulted, later blaming the sun. Suddenly, he lost the game, then the set, in a tiebreak. Del Potro went to the locker room, and when he returned, he sat down and covered his face with a towel. Fearing that might not be enough to hide how upset he was — this was a match a member of the Big Four would not lose — he put his towel-covered face in his hands.

“He’s coming apart mentally,” Cahill said in the television booth.

“And this is as methodical a player as there is on the tour,” Gilbert added. “There doesn’t appear to be any strategic thinking in the way he’s playing now.”

Del Potro quickly lost his serve in the third set and never came back. At one point, he walked up to an on-court television camera, blew a raspberry, then covered the lens with his hand. On match point, del Potro dumped a backhand in the net. “Good luck in New York,” Isner said when they met at the net. “I’ll see you there.”

It’s become something of a parlor game in tennis circles to speculate about the sport’s future once Roger Federer and Rafael Nadal, and eventually Andy Murray and Novak Djokovic, call it quits. Who’s next? When I first mentioned my interest in del Potro to a spokesman for the ATP, he was quick to suggest some other rising players. The tour knows it will not be able to milk the Federer-Nadal rivalry forever — their appearance in the quarterfinals at Cincinnati, which Nadal won, had a valedictory air — and it’s trying to lay the groundwork for a new generation of players that might look more like the current state of the women’s tour, with greater parity between the two or three dominant stars and their rotating cast of challengers.

Among the top contenders: Milos Raonic, a big-serving Canadian; Jerzy Janowicz, a big-serving Pole; and Grigor Dimitrov, a Bulgarian whose Federer-like backhand has earned him the nickname “Baby Fed.” Each is 22 years old, with considerable talent and rankings on the rise, but their results have been too inconsistent to predict which of them might jump to greater heights: Both Janowicz and Dimitrov lost their first-round matches in the U.S. Open this week. “Some good young kids need to start giving [the Big Four] a challenge,” Jimmy Connors told the New York Times recently. “And say what you want about it, but a couple of them should be American, too.” Don’t expect much on that front just yet: Isner is the highest-ranked American, at no. 17, and one of only two among the world’s top 80 players.

Del Potro is two years older than the fleet of 22-year-olds hovering in the rankings’ teens, 20s, and 30s, but he remains a likely candidate for long-term success, perhaps even domination. Four years after his Slam victory, he is still the youngest player in the top 10. (When I mentioned to Cahill that Del Potro was just 24, he was shocked: “He seems like he’s been around forever.”) The match against Djokovic at Wimbledon had served both as a reminder to the public of del Potro’s existence and as a boost to their affections. He is among the more likable players on the tour, his game is often electrifying, and he will have the New York crowd behind him in most every match he plays.

Del Potro won his first match Wednesday in four sets. He showed some rust after a lengthy rain delay, but for the most part he breezed through Round 1. “I think I’m getting closer to the top guys,” he said afterward. “That’s what I want.” There are no obvious threats in del Potro’s draw until the quarterfinals, where a likely rematch with Djokovic awaits. The Open begins with questions about Djokovic’s form and Federer’s age. Del Potro prefers hard courts, which makes this Open a reasonably likely one for his second title run. “I know how strong my game is,” del Potro told reporters before the Open. “I know good ways to beat them. But I need to work harder than them to be ready and wait for my opportunity.”

When del Potro won the Open, he was paraded around Tandil on a fire truck, in front of 40,000 people. Earlier this year, his hometown unveiled a mural on Avellaneda Street featuring del Potro in his Open-winning yellow headband. The city has already commissioned a sculptor for a statue to be completed at a date to be named later. All it needs now is a reason to be installed.