0232 GMT September 19, 2019
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, Reuters reported.
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.”