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.
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.”
“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.