MASON, Ohio -- Naomi Osaka felt a strange sensation in her left knee, different than anything she had experienced. The US Open champion left the court worried about whether she'd be in shape to defend her title.
Osaka retired from her quarterfinal match Friday at the Western & Southern Open, unsure of what's going on in her knee. She said she expected to get an MRI.
For now, the player currently ranked No. 1 in the world is concerned about her chances of making another deep run in New York.
"Last year, I won the US Open," she said. "This year, I'm trying to play the US Open."
The problem came out of the blue. Osaka split two sets with Sofia Kenin. She felt something unusual in the knee while serving early in the third set. After a timeout to have it examined and wrapped, she played a few more points before retiring.
"I have no idea what's going on," she said.
With that, Kenin advanced to the semifinals for the second consecutive week. During her run to the semifinals in Toronto, she knocked off then-No. 1 Ashleigh Barty, an upset that allowed Osaka to move up to the top ranking. She'll face Madison Keys, who beat Venus Williams 6-2, 6-3 to reach the semifinals for the first time.
Barty is in position to snatch No. 1 back heading into the Open. She beat Maria Sakkari 5-7, 6-2, 6-0 with an encouraging all-around effort.
"Still not quite there, but we're getting better and better each match," the French Open champion said. "I think that's the beauty of being able to stay alive in the tournament is to give myself an opportunity to keep getting better. I think I'm still aiming to obviously do very well here, but the big picture is New York in 10 days' time."
She needs some work on those first sets, though. Barty went ahead 5-2 in her opening set and got sloppy, letting it slip away. It was the second straight day Barty rallied after dropping a first set. She fought off a match point during a three-set win over Anett Kontaveit on Thursday.
"There is certainly no panic, where it gets to the stage where I'm kind of overwrought or can't think straight," she said. "I'm still able to, more times than not, have the ability to relax and kind of stay clear and come back to what works."
Kuznetsova is playing only her ninth event after missing the start of the season because of a knee injury. On a hot, humid afternoon, she got treatment for tightness in her upper right leg after the second set and twice during the final one.
On the men's side, Novak Djokovic continued his push for a second straight Cincinnati title, beating Lucas Pouille 7-6 (2), 6-1. He'll face Daniil Medvedev, who knocked off qualifier Andrey Rublev 6-2, 6-3.
Djokovic winced on a serve in the second set and got the muscles above and below his elbow rubbed during the changeover as he led 4-1. He broke Pouille and served it out despite the discomfort.
"I managed to finish the match," Djokovic said. "A little bit of tightness in my elbow, but hopefully everything will be fine tomorrow."
Qualifier Yoshihito Nishioka withdrew because of illness, sending David Goffin into the semifinals. Nishioka had a good tournament showing, beating sixth-seeded Kei Nishikori -- a player he called his hero -- and Alex De Minaur to reach the quarterfinals.
Goffin will face Richard Gasquet, who advanced with a three-set victory over Roberto Bautista Agut. Gasquet underwent groin surgery in January and fell to No. 68 in the ATP rankings July 29, his lowest spot in nine years. He's back in an ATP Masters 1000 semifinal for the first time since 2013 in Miami.
Seven-time champion Roger Federer was upset Thursday, leaving Djokovic as the only high seed left in the bracket.