(CNN) With the US Open just days away, Naomi Osaka is fretting over a knee injury after she was forced to retire from the Cincinnati Masters on Friday.

Osaka, who is the first player from Japan to win a major and climb to the No. 1 ranking, quit in the third set of her quarterfinal match against American Sofia Kenin.

"It' sucks, especially since I didn't want to get injured this close to the Open," US Open champion Osaka told reporters. "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."

Kenin won the first set, before Osaka leveled, but early in the third the Japanese star was seen by the trainer for nearly 10 minutes.