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World No.1 Naomi Osaka in action against Sofia Kenin, of the United States, during their Cincinnati quarterfinal on Friday. Image Credit: AP

Cincinnati: World number one 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 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 said she “felt it wasn’t safe” to continue against Kenin ahead of the US Open starting Monday week.

“I feel like this might have that one percent 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 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.

The Australian had surged to a 4-1 lead in the first set before she stumbled with unforced errors, with Sakkari lifting to take control through a slew of backhand winners.

World No.1 Novak Djokovic wore down Lucas Pouille after an even first set, dispatching the Frenchman 7-6(2) 6-1 in a quarter-final.

The match was much closer than when the pair met in the Australian Open semi-finals in January, when Djokovic conceded only four games before going on to win the title.

He will have less than 24 hours to recover for a semi-final against Daniil Medvedev, who continued his recent hot form, ousting Russian compatriot Andrey Rublev 6-2 6-3 in barely an hour.