Top-ranked Novak Djokovic was stunned in the semifinals of the U.S. Open, losing to Japan's Kei Nishikori 6-4, 1-6, 7-6 (4), 6-3 on Saturday. Nishikori became the first man to reach a Grand Slam final.

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"He just played better in these conditions than I did," Djokovic said, citing the intense New York heat as a factor in the upset.

At one point a thermometer on the court at Arthur Ashe Stadium showed the temperature close to 100 degrees (37 celsius), not counting the humidity which was near 70 percent. The match lasted two hours and fifty-two minutes.

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Despite playing in five-set marathons in his last two matches that totaled over eight and a half hours, Nishikori handled the heat well and looked like the fresher player throughout the match.

"It's just amazing, an amazing feeling beating the number-one player," Nishikori said during an on-court interview.

The 10th-seeded Nishikori will face the winner of Roger Federer vs. Marin Cilic in the title match on Monday. Nishikori cited the coaching of Michael Chang, the 1989 French Open champion, saying his mental game has improved to allow him to win matches like these.

"We've been working super well," Nishikori said of Chang and co-coach Dante Bottini. "That's why I'm here."

Djokovic was having a hard time throughout the match and spent most of it scrambling across the court. In the third set tiebreaker, he had four unforced errors and a double-fault. Djokovic was just 4-for-17 on break points, while Nishikori converted 5 of 7.

"Other than that second set, my game today was not even close to what I wanted it to be," Djokovic said. "A lot of unforced errors, a lot of short balls."

Chang was born in New Jersey to Taiwanese immigrants. At 17 he won at Roland Garros to become the youngest man to win a Grand Slam title.