TOKYO - Japan's former world No. 4 Kei Nishikori lost in the first round of the final tournament of his career at the Japan Open on Thursday, falling to Frances Tiafoe of the United States, 6-4, 6-4.

The 36-year-old Nishikori became the first Asian male player to reach a Grand Slam singles final at the 2014 U.S. Open, where he beat Novak Djokovic in the semifinals. He announced in late April that he would retire at the end of this season.

"I wasn't thinking I'd cry," a tearful Nishikori said after the match. He has been beset by injuries since undergoing left hip surgery in January 2022, including left knee and right shoulder problems following his return to competition in June 2023.

"But having begun playing tennis at five and realizing what I've accumulated over these 30 years is coming to an end here, that I won't be playing seriously anymore, I felt the sense of loss wash over me. Sadness is the overriding feeling."

Nishikori had a double fault to give up three break points before losing the opening game at Ariake Colosseum. Tiafoe's erratic first serves saw Nishikori get a break himself to tie at 2-2 but Tiafoe broke again to lead 4-3 en route to winning the first set.

Tiafoe, ranked eighth in the world, broke first again in the second set to lead 2-1 and the pair held serve until the current world No. 436 Nishikori, who produced several angled returns to the corners and led 6-2 in aces, went up 30-0 while trailing 5-4.

But the Japanese star could not make the chance count as he lost the next four points that ended the contest, failing to reach Tiafoe's drop shot to exit for one last time after 1 hour, 14 minutes.

"I think I tried hard in my last match, but it was nowhere near my ideal performance," Nishikori said. "I worked hard for the past half a year after returning from injury and came back to a decent level. But I'm frustrated to end this way."

"I must be someone who really hates losing as I wanted to show a bit more. But I'm happy I've stayed that way. I'm proud to have overcome my injuries and managed to play until the very end."

The Shimane Prefecture native moved to the United States in 2003, aged 13, to train at the IMG Academy in Florida. He joined the ATP Tour in 2007 and went on to win 12 titles, including two at the Japan Open in 2012 and 2014.

Nishikori reached world No. 4, the highest ranking for a Japanese man in the Open era, in March 2015. He beat Rafael Nadal to win bronze in the men's singles at the 2016 Rio de Janeiro Olympics, securing Japan's first games medal in tennis in 96 years.

"My honest feeling is I want to play a bit more, but my body, mentality and endurance aren't coming along," said Nishikori, who quipped he knows there is an ATP Challenger Tour event before the end of the year.

"I think I made a good choice in going to the United States to devote myself to tennis...There were matches that could have changed my life had I won, and I can find a lot of things that I regret. But I strongly feel I've done well taking everything into account."