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.
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With her knee heavily taped Osaka played one more game before retiring.
“My pain tolerance is really high, that’s usually why I play through things that apparently I shouldn’t,” added Osaka.
“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.”
In September 2018, Osaka won her first grand slam title, beating Serena Williams in a controversial US Open final that saw the American docked a game after calling the umpire a “thief.”
“Last year I won the US Open and this year I’m trying to play the US Open,” said Osaka, who secured a second grand slam title at the Australian Open in January.
“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% chance of me not playing it, that’s what’s concerning me.”
The US Open starts on September 2016.