'It wasn't safe': Naomi Osaka's US Open defense in doubt after knee injury picked up in Cincinnati

Japanese star Naomi Osaka's participation in this month's US Open has been called into question after the defending champion suffered an injury to her left knee which forced her to retire from her Cincinnati WTA quarter-final.

The 21-year-old was in obvious discomfort there, handing victory to American opponent Sofia Kenin and casting doubt on whether she will be able to return to Flushing Meadows to defend the first Grand Slam win of her career.

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After losing the first set to Kenin, Osaka stormed back in the second to claim it 6-1 but in the third she was forced to receive lengthy treatment to her knee from her trainer. She returned for one more game with her knee heavily bandaged but was forced to concede the match shortly after with her movement very much impaired.

"It sucks, especially since I didn't want to get injured this close to the open," Osaka said afterwards. "And now I'm kind of worried a little bit. The injured part sucks but losing, it's not that big of a deal. She was playing well.

"My pain tolerance is really high, that's usually why I play through things that apparently I shouldn't," she added. "I really don't know what's going on with my leg right now. I was asking the trainer if it was safe to play, because I really hate withdrawing.

"I went out there, I wanted to finish the set. But I felt this... like it wasn't safe."

Osaka was keen to point out that she isn't ruling herself out of contention at the tournament –which begins on August 26– until she learns the full extent of the injury. 

The Japanese player became a sporting sensation in last year's tournament when she defeated multiple winner Serena Williams in the final. She followed that up with another Grand Slam victory at the Australian Open in January, but her form has dipped in the intervening months.

But it's clear from Osaka's comments that she desperately wants to take part in the upcoming tournament in the United States.

"I don't even really think about winning the tournament," she said. "I just want to have the chance to play it now. If there is a one per cent chance of me not playing it, that's what's concerning me."