Sinner, who was already guaranteed a place in the last four after wins over Novak Djokovic and Stefanos Tsitsipas, won 6-2 5-7 6-4 to secure his first ever victory over Rune and top spot in the Green Group.
“For me, it was really important resetting after the good win against Novak,” Sinner said, per the ATP. “I never won against [Rune], so I really tried my best. I started off really well.
“In the second, he served better, he moved better, so it was a much more even match. The third set could have gone his way.
“At 4-3, I saved a break point. It was a little bit of a roller coaster today, but obviously really happy and happy to be in the semis.”
Another player who will have likely celebrated Sinner’s victory is Djokovic, who needed Sinner to win in order to reach the semifinals.
The 24-time grand slam singles champion beat Hubert Hurkacz 7-6 (7-1) 4-6 6-1 earlier on Thursday but knew it wouldn’t be enough to reach the last four if Rune won later that evening.
If the world No. 1 was watching the match unfold, his nerves will have likely been slightly on edge as Sinner appeared to begin struggling with a back injury deep into the third set.
However, cheered on by a raucous home crowd inside Turin’s Pala Alpitour arena, Sinner held serve to see out the match.
The win also means the 22-year-old becomes the first Italian man in the Open Era to record 60 victories in a season.
“It means for sure more doing it here in Italy, in Turin, a special place, special tournament,” Sinner said of the milestone.
“I’m very happy that I have the chance to have at least two more matches. Hopefully, I can finish the year in a good way. I think it’s going in the right direction, and let’s see what’s coming.”
In the Red Group, Andrey Rublev is already eliminated but Carlos Alcaraz, Alexander Zverev and Daniil Medvedev can all still qualify.
Alcaraz will play Medvedev in Friday’s afternoon session, before Rublev faces Zverev in the evening.