CINCINNATI, USA - World No.1 Naomi Osaka said she is 'worried a little bit' after picking up a knee injury that forced her to retire in the third set of her quarterfinal match against Sofia Kenin at the Western & Southern Open on Friday. 

The reigning US Open champion had come back from a set down to win six of the next eight games against Kenin, when she came down on her left leg after a serve in the second game of the decider and felt pain around her knee. After paying through the next point, Osaka sat down at 0-1, 40-40, and called the trainer. After a long evaluation and receiving some tape around the knee, Osaka played two points, did not move after her serve, was broken to 0-2, and retired.

Kenin advanced with a 6-4, 1-6, 2-0 (ret.) win, her second victory over a reigning No.1 in as many weeks. 

Speaking to reporters afterward, a concerned Osaka said the pain was like nothing she felt before. 

"The thing is, my pain tolerance is really high," Osaka said. "So that's usually why I play through things that apparently I shouldn't. 

"So I really don't know what's going on with my leg right now.

"I was asking the physio if it was safe to play, because I really hate withdrawing. I feel like we were playing such a great match, too. It's not fair to her to just withdraw, because I feel like I'm bowing out.

"Then I was asking the physio if she thinks it's safe to play. Then I went out there. I wanted to finish the set. But I felt it wasn't safe."

It was a frustrating end to a solid tournament for Osaka, who earned a confidence-boosting three-set win over Hsieh Su-Wei 24 hours earlier. Other than one poor service game in the opening set to hand over a break, Osaka's serve was on-song against Kenin. Osaka lost just five points behind her first serve all match, firing 10 aces, and saving 5 of 7 break points.

"It sucks, especially since I didn't want to get injured this close to the Open," Osaka said. "And now I'm kind of worried a little bit.

"She was playing well. I had to really think about what I had to do during the match, so I'm not that mad at the result. The injured part sucks, but losing, it's not that big of a deal."

Osaka will now turn her eyes on the US Open, which begins in 10 days on August 26th in New York. After making back-to-back quarterfinals in Toronto and Cincinnati, Osaka is hoping she gets an opportunity to defend her title.

"Last year I won the US Open, and this year I'm trying to play the US Open," Osaka said. "I don't even really think about winning the tournament. I just want to have the chance to play it, you know?

"I feel like this [injury] might have that 1% chance of me not being able to play, and that's what's concerning me. 

"In the end, I'm going to play even if the doctor says no, probably."