Cincinnati - Naomi Osaka's US Open title defence was in doubt on Friday after a left knee injury forced her to retire from her Cincinnati WTA quarter-final.
The Japanese star, who could lose her world number one ranking if Australian Ashleigh Barty wins her semi-final on Saturday, was forced to quit in pain - handing American Sofia Kenin a 6-4, 1-6, 2-0 win.
"It sucks, especially since I didn't want to get injured this close to the Open," Osaka said. "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."
Osaka had won the second set to level the match and was down a game in the third when she was seen by the trainer for nearly 10 minutes.
With her knee heavily taped she went back on court and played one more game before packing it in with her movement clearly impaired.
"My pain tolerance is really high, that's usually why I play through things that apparently I shouldn't," Osaka said.
"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 said she remained unsure of even what medical analysis she needs to determine the nature of the injury.
"Last year I won the US Open and this year I'm trying to play the US Open," she said of her mindset now with the US Open starting a week from Monday.
Osaka captured her first Grand Slam title at Flushing Meadows last year with a stunning upset of Serena Williams - whose mid-match tantrum made headlines.
Osaka backed up that triumph with a second Grand Slam title at the Australian Open in January, but she said now she's not thinking of a US title repeat.
"I don't even really think about winning the tournament. I just want to have the chance to play it. now.
"If there is a 1 percent chance of me not playing it, that's what's concerning me."