Stefanos Tsitsipas overcame Roger Federer in straight sets on Saturday and advanced to the final at the Nitto ATP Finals in London.
The Greek prevailed 6-3, 6-4 and will now meet either defending champion Alexander Zverev or Dominic Thiem on Sunday at the O2.
An energy-sapping first set went Tsitsipas' way after Federer failed to convert six break-point opportunities in the opener.
The burgeoning star broke Federer's serve at the start of the set to claim a 3-0 lead on his only break opportunity, and the 21-year-old showed resolve to frustrate his legendary opponent.
Tsitsipas closed out at 6-3, and Federer was clearly disappointed to have conceded a set he had gained traction in.
The 17-year age gap between the men appeared to favour the younger player, and Federer lacked his usual verve during crucial points.
Tsitsipas' ground strokes were at their best, and the world No. 6 stunned the crowd to repeat his earlier feat and break Federer's metronomic serve once again.
The early break in the second set handed Tsitsipas a 2-1 advantage, and Federer responded by increasing his pace and movement.
However, the Swiss lacked his usual accuracy, and he once again failed to take advantage when he seemingly had Tsitsipas on the ropes, continuing to waste multiple break points.
The crowd were awoken as Federer finally punished Tsitsipas to earn a break of serve, but Tsitsipas responded by immediately breaking Federer's serve to make it 3-2.
Federer displayed signs of fatigue in his footwork, with his body language visibly in decline between high-octane points. Tsitsipas carved out a 4-2 lead, taking him within two games from the final.
The 38-year-old Federer had little in the tank during the concluding moments, wasting two further break-point opportunities.
Tsitsipas proved his outstanding potential during a stunning victory, and in current form, he has every chance to become the champion.
Federer could only muster three aces during a disappointing display. He lacked the vitality to compete against the younger man.
Tsitsipas saved 11 break points, and this gave him the confidence needed to beat Federer during his debut entry at the tournament.