“Pressure” is a word that is used a lot in tennis. At times, it can seem less like a description of a stressful situation than a demonic external force, an invisible imp wreaking havoc on the court. Pressure pushed the ball an inch long. Pressure made it clip the net. Pressure shortened her ball toss. Pressure made him trip. Sometimes it can sound like skill and stamina are beside the point. When a player wins, he defeats pressure; when he loses, it is not to his opponent but to pressure that he succumbs.

And sometimes it really does come down to pressure. Tennis is hugely—and increasingly—physical, but it is also a game that’s played in the mind. Some players are easily influenced by pressure. First the feet slow, then the footwork completely goes. The swing tightens; the elbow drops. A player dumps an easy winner into the net, or sets up a point perfectly and then misses the open court. He makes bad decisions, or seems to swing without a thought at all about where the ball should go. Other athletes thrive under pressure; in fact, it is a big part of their success. Serena Williams’s serve gets better when she’s down a break point. Rafael Nadal’s forehand jumps higher off the court. And some players are masters of applying pressure. Andy Murray’s return makes servers nervous. Novak Djokovic can seem to hit every ball back just inside the lines, baiting opponents to try for too much.

Then there is Kei Nishikori, who has a more complicated relationship with pressure. The twenty-six-year-old, from Japan, currently ranked seventh in the world, is one of the few players who has consistently threatened the very top. During his run to the finals of the 2014 U.S. Open, where he lost to Marin Čilić, he beat Milos Raonic in a marathon match, then defeated Stan Wawrinka in another long contest, and then—when most players would have faded from so much time on the court—turned around and defeated Djokovic in the semifinals. Clearly, he knows how to play in big matches. In fact, by the A.T.P.’s own measure, Nishikori is one of the best pressure-situation players in history. According to the tour’s “career under-pressure ratings”—a statistic that combines the percentages of break points converted, break points saved, tiebreaks won, and deciding sets won (the third set for best-of-three matches and the fifth set for best-of-five)—Nishikori ranks fourth all-time, behind Djokovic, Pete Sampras, and Nadal. And yet Nishikori has a reputation for being someone who handles pressure badly. For a long time, he was known not for his deciding-set record but for having to play deciding sets in matches that he should have won easily. Usually so smart, he’d hit a loose shot. After making his first major final, he seemed to slide back, struggling with injuries and increased expectations.

Both Nishikori’s prowess under pressure and his weakness in the face of it were on display on Wednesday afternoon, in his U.S. Open quarter-final contest against Andy Murray—a thrilling five-set match that lasted nearly four hours. At the start, Nishikori seemed lost. He was rushing his shots, letting Murray—who has had a spectacular summer, winning Wimbledon and Olympic gold, and who was seeded second at the Open—dictate the points. “I felt like I didn’t know what to do,” he said after the match. His serve was abysmal, and his groundstrokes worse. He made fourteen errors in seven games, losing the first set 6–1. But a light rain began to fall partway through the second set: the players left the court, and the roof was closed. Nishikori changed his game, while Murray fixated on his sense of grievance at the distractions that swirled around him.

Part of the pleasure of watching a great player is that he expands the sense of how the game can be played. After the rough first set, what Nishikori’s serve lacked in speed was made up for with placement. He established patterns that were easy to see as they unfolded but hard to read in advance. He’d hit a serve out wide and then strike the ball cross-court, setting up a stunning forehand winner down the line. He’d jam Murray up the middle or punish him for shading into the alley by going down the T. He would counter Murray’s brutally consistent groundstrokes by serving and volleying or following shorter balls in, coming to the net thirty-nine times and winning nearly seventy per cent of those points. And he lured Murray in, often when the Scot was in the worst position to get there. Nishikori hit drop shots that died on the bounce, again and again. And while he is too slight to have superior power, he took time away from Murray by taking the ball early, hitting flat, and throwing his whole weight into his shots, leaping off the court and scissor-kicking his legs on some of his backhands. He dug out short balls and slid to cut off passing shots. He took more risks. Nishikori finished with sixty errors but forty-eight winners, while Murray had forty-six and just twenty-nine.

Murray, of course, has been playing the best tennis in the world lately, and often had the right answers. The match went back and forth, each player pushing the other. There was one swing that stood out. Nishikori, up a break in the fifth, was ahead 4–3, 40–love. But he hit two loose groundstrokes and then dumped an easy forehand volley into the net. Murray won the game, and it was hard not to think that Nishikori had faltered under pressure—that the more experienced Murray now had the edge. But Nishikori broke back and won the match 1–6, 6–4, 4–6, 6–1, 7–5. He will face Wawrinka in the semifinals today.*

For years, Nishikori’s talent has been obvious. He is one of the most dynamic shot-makers on tour, fast and agile, with perfect strokes and incredible hands. Yet, despite his skill and high ranking, when the majors come, he’s rarely counted among the spoilers, let alone the favorites. Last year, he lost in the second round of Wimbledon and in the first at the U.S. Open, as a defending finalist. This year, he won the bronze medal at the Olympics, but he only made it to the quarter-finals of the Australian Open and the fourth rounds at Roland Garros and Wimbledon. People are quicker to mention Raonic as the next slam champion, or younger players like Nick Kyrgios and Dominic Thiem.

Part of this has to do with Nishikori’s own insistence, when the attention was first on him, that he wasn’t quite ready to be at the top, even though he seemed to have the game to get there. After losing to Nadal at the 2014 Australian Open by the tightest of scores, he said, “I’m still a long way from winning against him.” This caution on his part may be connected to the extreme expectations he faces at home. In 2011, after he broke into the top hundred, he was known as “Project 45”: no Japanese player had ever been ranked higher than that. “He was having such a hard time,” his coach, Dante Bottini, told the Washington Post last year. “You could tell he felt the pressure so much. It was always, ‘If you win this match, you’re going to break the Project 45’—boom. He was 6–2, 2–0 up—boom, he loses 7–6 in the third. In the final, against this guy he should have beat—boom, choked so bad, lose. It was a nightmare." Nishikori has come a long way since then—he has spent the past couple of years ranked in the top ten—but the attention on him in Japan can still be overwhelming, and he lives in Florida to avoid it. “If I lived in Japan, it might get crazy,” he said earlier this year.

It may be, though, that Nishikori was right: he wasn’t ready then. Pressure? It is what you make it. After beating Murray, Nishikori put his hands over his head in triumph. He didn’t leap or fall to the ground as if he’d won a match of great consequence. That moment is still to come. He may yet face a vulnerable Djokovic, who was beaten at Wimbledon, lost an emotional match at the Olympics, and has been hampered by an injury to his left wrist. After two retirements and a walkover, Djokovic has had the chance to rest the injury, but he comes into the semis with little match play in the tournament. Djokovic is the best player in the world, but Nishikori has beaten him in the semifinals in New York before. The pressure is on. Watch out. It might get crazy.

*An earlier version of this post misidentified Nishikori’s opponent in the semifinals today.