Mason, Aug. 17 -- Naomi Osaka felt a strange sensation in her left knee, different than anything she'd experienced. The U.S. Open champion left the court worried about whether she'll be in shape to defend her precious title.

Osaka retired from her quarterfinal match at the Western & Southern Open on Friday, unsure what's going on in the knee. She expected to get an MRI.

For now, the player currently ranked No. 1 in the world is concerned about her chances of making another deep run in New York.

"Last year, I won the U.S. Open," she said. "This year, I'm trying to play the U.S. Open."

The problem came out of the blue.

Osaka split two sets with Sofia Kenin. She felt something unusual in the knee while serving early in the third set. Af...