SINGAPORE (AP) — U.S. Open champion Naomi Osaka finished her debut appearance at the WTA Finals on Friday with an injury and an 0-3 record.
Osaka was trailing Kiki Bertens 6-3 when she retired from the match with an upper left leg injury.
Bertens was awarded a straight-set victory and has advanced to the semifinals.
"I hurt my left hamstring in the first match," Osaka said. "It was just getting worse and worse every time I played.
"I just really wanted to try because this is the last tournament, and I felt it was really unfair to everyone that came to watch."
Sloane Stephens also advanced to the semifinals, improving to 3-0 by beating Angelique Kerber 6-3, 6-3.
Osaka, the only woman from Japan to win a Grand Slam title, went off court to receive treatment on the injury after the eighth game while trailing 5-3. She lost her serve at love in the ninth game and then decided to stop.
The 21-year-old Osaka, who started the year ranked No. 68 and is now at a career-high No. 4, beat Serena Williams in the U.S. Open final and watched as the 23-time Grand Slam champion argued with the chair umpire and was penalized for it.
"It's been a crazy year," Osaka said. "For me, it's just been a lot of new experiences. I'm very grateful that I was able to have the opportunities that I had."
Bertens qualified for the season-ending WTA Finals when No. 1 Simona Halep withdrew from the tournament with a lower back injury.
"That's how life can go sometimes," said Bertens, who has beaten 12 top-10 players this season. "I think I was a little lucky, maybe."
In the semifinals, Bertens will face Elina Svitolina while Stephens will take on Karolina Pliskova.
This year is the first time since the round-robin format was reintroduced in 2003 that none of the top four seeded players — No. 1 Kerber, No. 2 Caroline Wozniacki, No. 3 Osaka and No. 4 Petra Kvitova — reached the semifinals.
"I think the four of us, in our groups, we all played really well," Stephens said of the semifinalists. "I think it's great for us, and for the younger girls, to kind of prove ourselves."
More AP tennis: https://apnews.com/tag/apf-Tennis and https://twitter.com/AP_Sports