MASON     -    World number one Naomi Osaka will “probably” defend her U.S. 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 quarter-final 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 number two Ash 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’s retirement sent Kenin through to face fellow American Madison Keys in Saturday’s semi-final. 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 semi-final after rallying to a 5-7 6-2 6-0 victory over Greek Maria Sakkari.