Stefanos Tsitsipas produced a scintillating performance to beat rival Daniil Medvedev for the first time in a battle of two ATP Finals debutants at the O2 Arena.
Medvedev had won all five encounters with Tsitsipas before the Greek finally came out on top on Monday, winning 7-6 (7-5) 6-4.
Tsitsipas had a spring in his step from the start against an opponent whose game he has described as "boring" and did not face a solitary break point in the Group Andre Agassi opener.
Just the one break in the second set was enough for a fired-up Tsitsipas to seal a straight-sets victory over the fourth seed from Russia, who has been outstanding this year but faces a tough ask to reach the last four in London.
Tsitsipas started on the front foot and forced a break point in the second game, but Medvedev held to level at 1-1.
That was the one break-point opportunity of the opening set, and it was Tsitsipas who then came out on top in a tight tie-break, punching a volley on the line for a third mini-break and letting out a roar as his opponent netted on the first set point.
There was no let-up with the high tempo in the second set as the two continued to trade blows from the baseline and demonstrate their prowess at the net.
Medvedev fended off two break points before holding for a 4-3 lead, but he gifted a relentless Tsitsipas a break point when he left a ball which landed in, and an errant backhand left him 5-4 down.
Powerful sixth seed Tsitsipas jumped for joy after serving out a match in which he won 89 per cent of points behind his first serve.