Novak Djokovic expressed concerns about his right knee after experiencing pain during his 6-3, 7-6 (3) victory over Stefanos Tsitsipas, advancing to the Olympic semifinals for the fourth time in his career.

“I’m concerned about the state of the knee. I cannot give you exact information, because I don’t have it. I have to go and examine the knee now with my physio and with the medical staff of the tournament. And then let’s see,” said Djokovic, a 37-year-old from Serbia who will face Lorenzo Musetti of Italy on Friday.

Djokovic said he felt “sharp pain” early in the second set against Tsitsipas at Court Philippe Chatrier, the same stadium where he originally tore his meniscus on June 3 during a match at the French Open, forcing him to withdraw from that tournament.

He had an operation in Paris on June 5 but returned to action less than a month later at Wimbledon, where he wore a gray sleeve over that knee and played well enough to make it all the way to the final before losing to Carlos Alcaraz.

On Thursday, still wearing that sleeve, Djokovic was visited by a trainer while trailing 3-0 in the second set, then took anti-inflammatory pills a doctor gave him when the score was 4-1.

The medicine helped matters, Djokovic said, “But that effect is going to fade away tomorrow morning. So I will have probably a more realistic picture tomorrow and hope for the best.”

For all he has accomplished—more major championships than any other man in tennis history; 98 total trophies; more weeks at No. 1 than anyone else since the computerized rankings began a half-century ago—the one significant item Djokovic is missing from his resume is an Olympic title (he won a bronze at Beijing in 2008). And he made clear that was a priority this season.

Djokovic did not lose a set through his first three matches at the Games, including a 6-1, 6-4 victory over rival Rafael Nadal.

Against Tsitsipas—a 25-year-old from Greece who twice was the runner-up to Djokovic in two major finals, including at the 2021 French Open—there were no apparent issues during the opening set. On that set’s last point, Djokovic slid to his right to smack a cross-court forehand winner, eyes wide, then waved his arms overhead to encourage the crowd to get rowdier.

But in the second set, he took an awkward step on one point, then another. He was grimacing and wincing and limping occasionally. His wife, Jelena, looked on anxiously from the stands while holding a small Serbian flag. His team appeared worried, too.

Things didn’t seem great on the scoreboard, either.

Tsitsipas served for the set at 5-3, earning three chances to force a third when he went up 40-love in that game. But Djokovic, as resilient as anyone, did not yield.

He broke there and, as usual, was superior in the concluding tiebreaker.

“I feel like I could have done so much more today,” Tsitsipas said. “Novak is one of the best players in the world. He … did give me a lesson: I should be much more into the match whenever I have the opportunity to close it, and today I didn’t show that.”