Novak Djokovic admitted he was "very worried" about his fitness after having to take two medical timeouts in his victory over Stefanos Tsitsipas on Thursday.

The Serb won 6-3 7-6 (7-3) in their quarter-final meeting but was visibly struggling with the knee injury that forced him to withdraw from the French Open.

During a lengthy second set, Djokovic came from 5-2 down to force a tie-break, saving set point three times, but he also needed to be seen by his trainer twice as the match took its toll.

And after the match, Djokovic was left feeling anxious over the condition of his knee.

"I am very worried, honestly, I don't know what to say," Djokovic said, via AFP. "I finished the match, and the intense pain went away. Whether I hit myself or slipped, I don't even know.

"It all resembled the fourth round of Roland-Garros where I managed to finish the match, and then I found out I was injured. I am pleased with the victory, but I am worried."

Djokovic, who is chasing his first-ever Olympic gold medal, had already confirmed he would be talking to his medical team after expressing his concerns.

If he is able to play, he will face Italy's Lorenzo Musetti, who beat defending champion Alexander Zverev, in the semi-final. The pair faced off at Wimbledon last month, with the former world number one coming out on top in straight sets.

"It's day-by-day now," Djokovic said.

"Musetti won against Zverev today, he’s in terrific form. [He’s] played at least semis in every event, I think, for the last five, six events.

"We had an incredible late-night finish here in the third round of Roland-Garros - I think it was up to 3am we played - so I hope I can be physically fit for that one because that's what's going to be necessary to try and get the win."