TURIN, Italy (AFP) — Novak Djokovic stayed on track for a record-breaking seventh ATP Finals title after Jannik Sinner beat Holger Rune 6-2, 5-7, 6-4 to seal the Serbian world No. 1's  semifinal ticket on Thursday.

Sinner was already guaranteed a spot in the Last Four after Hubert Hurkacz claimed a set in his 7-6 (7/1), 4-6, 6-1 defeat to Djokovic earlier in the day, and the world No. 4 topped the group after a battling win at the Pala Alpitour in Turin.

"It was hard, I'd never beaten him before, the crowd helped me a lot," Sinner said.

"I'm pleased with how I won the game."

With Sinner already assured of a semifinal berth, the Italian could have lost against Rune knowing that a defeat would eliminate Djokovic from the tournament.

But Sinner fought hard to secure first place with his third win in three group matches, allowing Djokovic to follow him into the next round.

In the semis, the reigning Toronto Masters champion will play whoever finishes second in the Red Group, while Djokovic will face the group winner.

Carlos Alcaraz and current Red Group leader Daniil Medvedev, who is also already in the semis, face off on Friday afternoon before Alexander Zverev takes on eliminated Andrey Rublev.

Sinner initially looked like making short work of Rune despite the Dane only having to play three games in his previous match with Stefanos Tsitsipas on Tuesday before the Greek dropped out of the tournament injured.

The 22-year-old came through a three-hour thriller against Djokovic but looked ready to take on the world when he broke Rune twice in the first three games.

Fatigue and a back niggle allowed Rune back into the match, but Sinner held firm to bask in the cheers of fans who chanted his name to the rafters.

"My back is okay, I felt a small pain but it's gone now," Sinner added.

"It might have just been the tension of the match, I didn't even call over my physio. I'm not worried."