Knee injury puts world No. 1 Osaka's US Open title defence in doubt

World No. 1 Naomi Osaka will "probably" defend her US Open title, even if it means defying her doctor's orders, she said after retiring from the Cincinnati Masters with a knee injury on Friday.

Osaka was trailing American Sofia Kenin in the third set of their quarterfinal in Cincinnati when she sought treatment on her heavily-strapped left knee.

After briefly returning to play, the Japanese star retired with Kenin leading 6-4, 1-6, 2-0.

Osaka's withdrawal comes after Serena Williams, the woman she beat in a tumultuous final at Flushing Meadows last year, pulled out of Cincinnati with back problems.

The injury has also loosened Osaka's grip on the top ranking, with world No. 2 Ashleigh Barty poised to reclaim it if the Australian makes the final.

"I really don't know what's going on with my leg right now," Osaka told reporters.

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

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

Osaka said she "felt it wasn't safe" to continue against Kenin ahead of the US Open starting on August 26.

"I feel like this might have that one per cent 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."

Osaka's retirement sent Kenin through to face fellow American Madison Keys in Saturday's semifinal.

Keys faced only two break points in beating Venus Williams 6-2, 6-3 in the evening encounter.

Barty will face Russian veteran Svetlana Kuznetsova in the other semifinal after rallying to a 5-7, 6-2, 6-0 victory over Greek Maria Sakkari.

Kuznetsova outlasted Czech third seed Karolina Pliskova 3-6, 7-6 (2), 6-3, having been a set down with her opponent serving for the match.