Kei Nishikori tops 2014 U.S. Open champion Marin Cilic to reach U.S. Open semifinals.

Wednesday was another day of U.S. Open players functioning as lab rats in some sort of heat-tolerance experiment. Temperature heat and intensity heat. Another day of testing their ability to concentrate on the moment — no matter the weather, the score, the opponent, the past, the crowd.

    “It wasn’t easy, but I tried to stay calm. And I really focused,” said Japan’s Kei Nishikori, playing Croatia’s Marin Cilic in a rematch of their 2014 Open final.

    Their Wednesday quarterfinal duel lasted more than four hours, five roller-coaster sets during which Cilic lamented that he had “just a little bit lost focus” at times, eventually spelling his doom in a 2-6, 6-4, 7-6 (5), 4-6, 6-4 loss.

    Focus. That Cilic had won that 2014 title match had to be dismissed. “Just focus what I have to do today,” he said. “That match was four years ago, so I feel we are both different, we both need to bring the tennis and bring the really good game to the match. It’s not that just psychologically, it’s going to be taken care of.”

    Throw out the seedings. Nishikori was No. 21, Cilic No. 7. Forget the fact that Cilic had won two of their three major-tournament meetings. Move on from the last point. Pay attention to right now.

    “I came out great_6-2, 4-2 serving, and everything was going great,” Cilic said. “Then just a little bit shift of the momentum there. Kei started to hit a little bit better. He won six games in a row there, converted three of three break points. Just put me a bit on my back foot.”

    Cilic entered the proceedings with more career success. Besides the ’14 U.S. title, he was runner-up in two of the last five Grand Slam events—Wimbledon ’17 and this year’s Australian Open. But that wasn’t going to help him Wednesday.

    “You know, in some games,” Cilic said, “I would connect and play three, four, five, six points good and create myself opportunities. And then I would waste it.”

    Nishikori, in 35 previous major tournaments, had made it as far as the semifinals only twice, both at Flushing Meadows—in 2014 and 2016. But that wasn’t going to change his chances Wednesday, any more than having to skip the 2017 Open with a torn wrist tendon.

    The task was to lock into the day’s demanding baseline play in temperatures that felt to be in the mid-90s. Of the two players’ 296 points won, only 27 were scored from the net.

    “It wasn’t easy,” Nishikori said. “But I fought through somehow. And especially after coming from injury, I think I’m enjoying this challenge.”

    No Japanese player ever has won a major-tournament singles title, though Nishikori came the closest four years ago and, on Wednesday, he and  20th-seeded Naomi Osaka advanced to the Open’s men’s and women’s semis. No Japanese player ever had been ranked in the world’s top five until Nishikori reached No. 4 in 2015.

    In 1995, Japan’s Shuzo Matsuoka, a Wimbledon quarterfinalist, reached No. 46 in the world. So when a 14-year-old Nishikori moved to Florida in 2004 to begin training at the Bollettieri tennis academy, he was billed as “Project 45” in a campaign to make him the highest ranked player in his nation’s history. His coach, Dante Bottini, has called that a “nightmare” for the young Nishikori, subjecting him to undue pressure.

    But he got past all that the way tennis players do what they do. With their eye on the ball and, especially, with Focus.