Naomi Osaka retired from her quarterfinal match at the Cincinnati Masters on Friday, due to uncertainty with her knee.

The 21-year-old Japanese is expected to know more after MRI scans. 

For now, the player, who currently ranked number one in the world, is concerned about her chances of making another deep run during the U.S. Open in New York.

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

Osaka was trailing American Sofia Kenin in the third set when the Japanese player sought treatment on her left knee.

After briefly returning to play, Osaka retired. Kenin was leading 6-4 1-6 2-0.

The withdrawal comes less than two weeks before the start of the U.S. Open, where Osaka is the defending champion.

"This isn't the way I wanted to win," Kenin said in a courtside interview.

"I wish Naomi a speedy recovery so she can get ready for the U.S. Open. She's a great player and I have a lot of respect for her."

Kenin advanced to the semifinals for the second straight week. 

During her run to the semifinals in Toronto last week, Kenin knocked off then-world number one Ashleigh Barty of Australia, an upset that allowed Osaka to move up to the top ranking.

The 23-year-old Australian is in position to snatch it back heading into the U.S. Open. She beat Maria Sakkari 5-7, 6-2, 6-0 with an encouraging all-around effort.

"Still not quite there, but we're getting better and better each match," said Barty, who won the French Open champion this year.

"I think that's the beauty of being able to stay alive in the tournament is to give myself an opportunity to keep getting better. I think I'm still aiming to obviously do very well here, but the big picture is New York in 10 days' time."

On the men's side, Japanese qualifier Yoshihito Nishioka withdrew because of illness, sending Belgian David Goffin into the semifinals. Nishioka had a good tournament showing, beating sixth-seeded Kei Nishikori – a player he called his hero – and Australian Alex De Minaur to reach the quarterfinals.

Seven-time champion Roger Federer of Switzerland was upset Thursday, leaving Novak Djokovic of Serbia as the only high seed left in the bracket.

(With input from AP and Reuters)