World No. 1 Naomi Osaka withdrew from the Italian Open ahead of her quarterfinal match on Friday before her compatriot Kei Nishikori was knocked out in straight sets.

Both of Japan's top players had won two games on Thursday, but Osaka said she was unable to move her right thumb when she woke up. Nishikori, the world's sixth-ranked men's player, made it to his quarterfinal match against Diego Schwartzman and fought back from 5-0 in the first set before falling 6-4, 6-2.

After letting Nishikori back into the first set, the 24th-ranked player took the air out of his Japanese opponent in the seventh game. Having broken in the sixth game to take a 4-2 lead, Schwartzman held serve in a superb seventh-game rally in which he saved three would-be winners from Nishikori.

His body fully extended and heading out of the court, Schwartzman hit an unbelievable winner that brought the crowd to its feet. The Argentine clinched the match in the next game when Nishikori double faulted.

Earlier, Osaka said she was not aware of any problem from the previous day's matches but was unable to move the thumb after waking up.

"I tried to practice and grip my racket, but I couldn't. Then I just kept feeling this pain every time I tried to move my hand in different directions," she said at a press conference.

"Right now I'm really not mad but I'm kind of in between sad and disappointed because I really wanted to play my match today. For me, it was sort of like a test to play against Kiki (Bertens) because she's playing really well. I wanted to see how well I could do today," she said.

Bertens, of the Netherlands and ranked fourth in the world, advanced to the semifinals at the elite event via a walkover.

"I didn't feel anything yesterday. That's why I'm kind of confused right now because I literally woke up in the morning and I couldn't move my thumb," said Osaka, who added she had seen a trainer but had not yet been to the doctor.

(Naomi Osaka (R) of Japan and Mihaela Buzarnescu of Romania)