At the 2006 U.S. Open, Andre Agassi was 36 years old and staring directly into his post-tennis life. That was his 21st tournament in Flushing, and it had become clear that Agassi’s body was failing him. The American had won a four-hour second-round match against eighth-seeded Marcos Baghdatis, but he collapsed in the parking lot later that night when his back started to spasm. His third-round match, and his career, ended decisively against Benjamin Becker, who was ranked outside of the world’s top 100 players. About a week later, a 25-year-old Roger Federer would win the tournament. Even for the immutable Agassi, it had become clear that tennis was a young person’s game.

Next year, Roger Federer will turn 36. In 2016, the Swiss played only 28 matches, but looked more than capable in the spotlight, reaching the semifinals of the two majors that he entered. Even now, Federer isn’t just “good for his age,” he’s good by almost any measure — by the ATP’s estimation, he’s better than all but 15 players in the world, and that’s after sitting out every tournament after this year’s Wimbledon and forfeiting more than half a year’s share of rankings points. This mid-30s resilience is, of course, not historically unprecedented; Agassi finished his penultimate season ranked seventh after reaching the final of the 2005 U.S. Open, losing to Federer. But, the next-oldest player to finish in the year-end top eight that year was 24-year-old Lleyton Hewitt. Today, 23-year-old Dominic Thiem is the only player younger than 25 ranked among the world’s top 10. Six members of that group are 29 and older, including the three players who won majors this year.

Federer seems like less a man out of time than did Agassi, who played against three distinct generations of players, because his competition has circled him for the last decade. Novak Djokovic and Rafael Nadal, both early bloomers and generational talents, have managed to maintain top form for more than a decade, but even the ATP’s set of secondary characters like David Ferrer (34 years old, no. 21), Jo-Wilfried Tsonga (31 years old, no. 12), and Tomas Berdych (31 years old, no. 10) have, in recent years, managed to continue losing in major quarterfinals as if their primes weren’t supposed to end years ago.

Related The Dichotomous Declines of Roger Federer and Rafael Nadal

There’s a simple, if narrow, way to explain this: Improved training and a golden group of players have been buoyed by a generation of 25-year-old losers. Most of us accept a narrative that, when distilled, sounds rather silly: Modern greats benefit from only stronger wills, better preparation, or some magical combination of the two. But many of the causes of this phenomenon have been obvious, if not heavily exposed, because they’ve been happening inside string beds, in concrete mixes, and in the choice of seeds that will end up sprouting on courts in June.

If you’ve ever played table tennis, chances are that you’ve noticed that the available paddles make a significant impact on the style and quality of play. Even experienced players can easily be handcuffed by the plastic monstrosities that usually accompany public tables. But, imagine that table tennis was always played with these paddles. The game that would develop would be flatter and geared toward finding a quick winner, as playing long rallies with flatline trajectories would carry a very high degree of difficulty. Very simply, with plastic paddles, the game would not advantage planning and endurance. Even the best tactician can do only so much while slapping a speeding ball in a straight line.

Now, imagine that some players begin using the foam and rubber-faced paddles that are used regularly in competitive and semicompetitive play. Suddenly, players can grip the ball, spin it left or right, make it deaden after a bounce or leap up higher than usual. They can still slap the ball as players did before, but now not only are more shots are available, more extreme plays are on hand as well. Now, with so many new trajectories becoming part of the game, more shots would stay in, points would extend. The ability to play the third and fourth and fifth shots in a rally would be critical, as would the endurance to do so repeatedly over the course of a match. The player with the biggest serve or forehand would be less consequential than the competitor who could consistently cover the table while playing every shot serviceably. For older, more aggressive players, this change would be loud and devastating. Yet, this is essentially what happened rather quietly in the mainstream tennis community around the beginning of the millennium due to the work of a string company called Luxilon.

For decades, tennis was played with “natural gut” strings made of cow intestines. These strings have generally fallen out of favor in the amateur community because of their high price and relatively low durability, as compared to synthetic nylon strings, which are cheaper but generally lower in quality. Of course, cost of equipment is of no concern at the top professional levels, but about 20 years ago, an equally inexpensive string produced by a company called Luxilon (which makes tennis strings, but also many materials not used for anything remotely related to tennis) made of polyester-like material started to gain traction on tour. This “poly” material dramatically increased spin due to decreased friction between strings. On contact with the ball, the strings slid out of their usual position, like their natural-gut counterparts, but then snapped back into place, dramatically increasing the rotation of the resulting ball. This may sound like the common marketing of a sports equipment company that suggests that its gear will give players an edge, but Luxilon’s tech worked, and tennis history has some strange happenings to show for it.

Gustavo Kuerten won three majors, though his success mostly occupied an overlap in less globally popular tennis spaces: the clay court season and the years around the turn of the millennium. In 1997, ranked 66th, the 20-year-old Kuerten upset the men who had combined to win the last four titles in Paris en route to winning his own French Open.

Though tennis was significantly less predictable two decades ago than it is now, Kuerten’s run was still surprising, especially given that few knew he was an early mover on poly strings. In the coming years, the Brazilian would move his way into the top 10 despite enjoying almost no success at any major besides Roland Garros. (The rest of the tour at this time also struggled to string wins together, so one major win in a year was plenty for a top player — until Federer came along and started winning everything around 2003–04.) Over three separate stints, Kuerten spent 43 weeks at no. 1, and the impact of his Luxilon-aided performances quickly became a point of attention on the tour. His increased margins and control made all the difference. Last year, Tennis looked back at the effects of Kuerten and his magic strings:

“By 2000, Kuerten was no. 1 in the world, and some of his fellow players began to whisper that his string should be banned for giving him an unfair advantage. Tour stringer Nate Ferguson has said that as he watched Guga dip passing shots at the feet of Pete Sampras on his way to the title at the Tennis Masters Cup (now called the World Tour Finals) in Lisbon in 2000, he wondered how anyone would ever be able to rush the net again.”

By the 1990s, many players had begun playing a “power baseline” game, which is basically top-level tennis as we see it today. Power baseliners — well, they hit powerful shots from the baseline. They weren’t quite defensive counterpunchers in the mold of Bjorn Borg, and they certainly weren’t serve-and-volley players who preferred never to hit groundstrokes at all. There likely is no more famous representative of the ’90s power baseline game than Andre Agassi.

David Foster Wallace, who called the power baseline game a “brutal art,” said of the style: “Agassi’s balls look more like Borg’s balls would have looked if Borg had been on a yearlong regimen of both steroids and methamphetamines and was hitting every single fucking ball just as hard as he could. Agassi hits his ground strokes as hard as anybody who’s ever played tennis–so hard you almost can’t believe it in person.”

At the time, this made Agassi unique, but today, this description could fit any player who could ever hope to win an important tournament.

For a time, serve-and-volley players still found success at the top levels of the sport. Sampras, six times the year-end no. 1 during the ’90s, Boris Becker, and the towering Croat Goran Ivanisevic, all of whom made their livings at the net, were fully capable of competing with power baseliners like Agassi over the course of their careers. But after 2000, older serve-and-volleyers started to retire, and perhaps, thanks to Luxilon, no younger net-rushers took their place. These magic strings could widen the gaps between the ball and the line, but they couldn’t shorten the distance between an aging volleyer and a speeding passing shot.

Previously, even the world’s best players relied on certain parts of their games (Sampras’s serve and forehand, Agassi’s returns) to win most of their points. But, the new power-baseline game, boosted by these strings, brought weak shots up to par and often neutralized previously lethal weapons. Attributes more easily trained and retained into the late 20s and 30s, consistency and endurance, became the must-have tools of a dominant player in a poly-infused game. Barring injury, a player can keep his fitness and groundstrokes at top levels with careful preparation. Even the best net players will lose some speed on their serves and milliseconds on their twitches at net, but a power baseliner, because his skill set is so wide, can be exceptional late into his career even if one or two facets of his game slightly decline.

Few modern players likely lost out due to the shift to the all-around game brought about by poly more than Andy Roddick, who sported a monstrous serve, a big forehand, and not much else. Roddick, after winning the 2003 U.S. Open, would never be able to serve his way to another major. Federer, and others later, could too easily get in front of Roddick’s serve before thumping him and his awkward backhand in the open court.

During the 2007 U.S. Open, a retired Agassi commentated on a quarterfinal match between Federer and Roddick. In the first set on consecutive points, the American hit screaming serves at 136 and 137 miles per hour, only to have both calmly blocked back by the world no. 1. “Roger’s ability to get his racket on these serves is incredible to me,” Agassi said. “You know, when a champion has two things they can count on out there as best in the world, they’re a dominant champion. I think Roger has closer to five.”

In most sports, competitive environments are strictly standardized. Athletics are built on instinctive knowledge of space. This awareness is critical with respect to both training and competition. When Syracuse played San Diego State in 2012 on the deck of an aircraft carrier, SDSU played their usual brand of basketball, and were rewarded with a 1–18 performance from 3. “It’s just like when you play outdoors in the summer,” said Syracuse head coach Jim Boeheim. “You’ve got to get to the basket. That’s why kids from the city don’t shoot jump shots.” Syracuse, on the other hand, took only four shots from 3. The Orange won by 13.

That San Diego State would approach a windy oceanside outdoor game the same way they would treat a game played in a temperature-controlled bubble seems ill-advised at best and completely irresponsible at worst. But this is a calculus basketball teams almost never have to make.

Tennis, played annually on varying forms of concrete, grass, and crushed brick in both indoor and outdoor arenas, is a bizarrely nonuniform sport. Each surface is markedly different: Clay, a defensive haven, dulls the pace of powerful strokes and causes balls to sit up; grass is lightning quick, a home for aggressive players known for producing skidding shots and uneven bounces; the U.S. Open is played on speedy DecoTurf hard courts while the Australian Open uses a slower Plexicushion surface. For decades, each tournament’s courts played at different speeds, but more recently, the speeds of the courts, and hence, the styles of successful players, have begun converging.

In an interview at the 2012 Madrid Masters, which was played on an experimental blue clay surface disparaged by top players, Federer said of court surfaces, “I came in a time, back in ’98, ’99, 2000, where indoors was brutally quick, clay was slow, grass was fast, and we had to adapt every single week, then the last, sort of, six, seven years, eight years, have all been pretty much the same.”

This shift to slower, similar courts is seldom spoken about on television and admitted by tournament organizers even less. But slower courts lead to a more marketable product. Top players suited to the median court speed will rack up historic numbers of titles, rallies will be longer and more obviously “exciting.” A Sampras-Ivanisevic ace-fest may have been an enjoyable experience for purists, but for the average watcher, Murray and Djokovic playing 30-shot rallies at full stretch is much more appealing. Except, who would want to admit that there were external forces creating this great product when a simple “we live in an extraordinary era,” would suffice?

“What used to be four radically different surfaces, requiring four radically different styles of play, have become increasingly homogenous,” Brian Phillips wrote for Grantland in 2013. “This is a major factor — arguably the major factor — in the current state of the game, particularly in men’s tennis, where it has helped shape both the Nadal/Federer/Djokovic/Murray golden age and the slow-paced, relentless, defensive tennis that more and more seems to define it. There’s a serious argument to be made that Rafa Nadal, Novak Djokovic, and even Roger Federer could never have won so many majors if they’d played in an era before the biggest competitions started rewarding similar skill sets.”

With court surfaces converging toward a slower pace, every tournament has now become, largely, the domain of the same players. And with more events becoming home to extended rallies, a player like Nadal no longer has to concede Wimbledon and the entire back half of the season to more aggressive competitors. Slower courts and more time to play ideal shots create more opportunities for the best tacticians. From there, the law of averages tells the rest of the story. Slower, similar courts simply increase a player’s margin for error. When athletes get older, bad matches or tournaments become more common, but when there are simply more realistic opportunities to succeed, a top player’s fall from grace can be slowed dramatically.

At this year’s World Tour Finals, which end this weekend, even with Federer (knee injury) and Nadal (who still qualified, but sat out due to a wrist injury) both absent, the average player in the O2 Arena is almost 28 years old. Half of the competitors are the same as those at the 2015 edition.

In the past, young players like Roddick or Hewitt could emerge and win tournaments by simply running faster and hitting the ball harder than their older counterparts. But as time has gone by, physical treatments have improved while the game has become more forgiving. It’s not visibly obvious what Andy Murray and Novak Djokovic, who are both still alive in the Tour Finals, do that separates them from the field. Rather, they get their edge from nuanced vision and an understanding of the game that young players like Thiem can’t just hit away.

It’s possible, if unlikely, that these players, almost invincible at an age that used to inevitably indicate a summer trip to the Graveyard of Champions, are only the arbiters of a temporary move away from the young person’s tour of years past. But improved medicine and training methods, gluten-free diets, poly strings, and slower courts aren’t going anywhere. Neither, it seems, are Djokovic and his contemporaries. Modern tennis, seemingly, is less a game of twitches and millimeters than it is a product of fitness and preparation. Maybe 30 really is the new 20.