Andrey Rublev produced a stunning come-from-behind victory to down Stefanos Tsitsipas 3-6 6-3 6-2 and reach the last four at the ATP Finals, while Novak Djokovic stayed unbeaten in the round-robin stage with a win over Daniil Medvedev on Friday.
Tsitsipas came out firing to dominate the first set, pouncing on a shallow ball from Rublev to crush a forehand winner for the early break and a 3-1 lead as the fiery Russian's temper began to boil over.
The Greek claimed the set when Rublev failed to put one of his second serves into play and it appeared the Russian might go quietly from there.
Instead, Rublev cut down on the errors in the second set and let out a mighty roar at Pala Alpitour when he broke Tsitsipas for a 5-3 lead.
Rublev saved a break point in the next game and held his fist up high when Tsitsipas' forehand went wide to level the contest at a set apiece.
From there it was Tsitsipas who struggled to keep his composure as Rublev continued to apply pressure, breaking for 2-1 in the third set and again for 5-2 on back-to-back double faults by the world number three.
"I just kept telling myself to keep fighting, keep playing, and we'll see what happens," Rublev said.
"I was happy that I was mentally strong today and I was able to turn the match around because Stef in the beginning was hitting super hard, hitting so many forehand winners."
Next up for Rublev is a semi-final against Norway's Casper Ruud.
GRUELLING CLASH
Earlier on Friday, Djokovic overcame Russia's Medvedev 6-3 6-7(5) 7-6(2) in a gruelling clash to top the Red Group.
Djokovic, who is chasing a sixth title at the season-ending championship to match Roger Federer's record, drew first blood in a tight opening set to take a 5-3 lead and consolidated the break for the early advantage in the match.
Trailing 5-4 in the next, the Serbian saved three set points and drew level with a blistering cross-court shot at the end of a 26-shot rally but appeared to struggle physically in losing the tiebreak as he allowed Medvedev back into the match.
Having already reached the semis after beating Rublev on Wednesday, some might have expected Djokovic to ease up and save his energy for Saturday's semi-final meeting with Taylor Fritz but the 35-year-old responded in style.
The Serbian battled back from a break down in the decider to hand Medvedev his third straight defeat after three hours and five minutes of thrilling tennis.
"Daniil and I have had some battles in the past and I knew coming into the match it was going to be his last of the season and he wouldn't want to finish with a loss," Djokovic said.
"I started off well, had chances in the second set, had break points but he played very well in those decisive points. He was playing really quick. I didn't feel physically well in the third, struggled a lot. It's a battle and it's a fight.
"Being able to find the last drop of energy to come back ... at 4-5 (in the third set) I managed to read his serve and anticipate well and got myself in a good position."