Novak Djokovic advanced to the semifinals of the Paris Olympics but admitted to being “very worried” about his surgically repaired right knee.

“I am very worried, honestly, I don’t know what to say,” he said following his 6-3, 7-6(3) quarterfinal victory over Stefanos Tsitsipas, 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 4th 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”

The 37-year-old Serb and 24-time Grand Slam singles champion trailed 2-5 in the second set. He saved three set points and twice called the physio for his right knee, the same knee on which he had surgery in early June after withdrawing from Roland Garros.

The No. 1 seed moved gingerly and grimaced in pain throughout the early stages of the second set. After calling the physio at 0-3 and again at 1-4, Djokovic appeared to regain confidence in his movement. He crushed a forehand winner while sliding towards the right, putting loads of pressure on his right leg, on a pivotal deuce point. Djokovic saved three set points on return at 4-5, 0-40.

Djokovic leads Tsitsipas 12-2 in their head-to-head. Thursday marked their first meeting since the 2023 Australian Open final won by Djokovic.

Up next for the top seed is No. 11 Lorenzo Musetti, who upset defending Olympic gold medalist Alexander Zverev 7-5, 7-5. Djokovic has won six of his seven meetings against Musetti, including three matches this season. Djokovic defeated Musetti in the Wimbledon semifinals.

Djokovic, who won the singles bronze medal at the 2008 Beijing Olympics, could meet rival Carlos Alcaraz in the gold-medal match.

No. 2 Alcaraz advanced with a 6-3, 7-6(7) win over American Tommy Paul.

Alcaraz, who struck 22 winners in his two-hour one-minute victory, will face Canadian Felix Auger-Aliassime in the semifinals after the he beat Norway’s Casper Ruud 6-4, 6-7(8), 6-3. Alcaraz is the youngest semifinalist in the men’s singles event since Djokovic in Beijing in 2008.

Alcaraz, who now leads Paul 4-2 head-to-head, is making his Olympic debut and will have the chance to compete for a medal. If he wins his semifinal he will play for gold and if he loses he will play for bronze.

“It’s not a secret that right now, Alcaraz is kind of like the golden standard,” Paul said. “You use him to go to the practice court and see what you can change the next time you play him, because everyone wants to play at his level right now. He’s won the last two Slams, he’s young, he’s full of energy. So if you want to be the best in the world, you’ve got to beat him. So we went from Wimbledon to the clay with Carlos in mind. We’re trying to be at that level.”

Alcaraz suffered a disappointing defeat in doubles on Wednesday evening alongside countryman and legend Rafael Nadal. The Spaniards lost to Americans Austin Krajicek and Rajeev Ram in straight sets but Alcaraz did not let that setback deter him against Paul.

(The ATP contributed.)