Stefanos Tsitsipas capped a dream week at the O2 by beating Dominic Thiem 6-7 (6) 6-2 7-6 (4) to capture the ATP Finals.
One day after beating Roger Federer, the Greek youngster completed his graduation to the upper echelons of the sport by taking revenge over Thiem, who came from a set down to beat him in Beijing last month.
It caps a memorable 12 months since he landed the Next Gen title, and now the 21-year-old will have his sights set on the Australian Open, where he reached the semi-finals in January.
Tsitsipas is the youngest winner of the prestigious season-ending tournament since Lleyton Hewitt in 2001, and the youngest debutant to lift the trophy since John McEnroe back in 1978.
The first set was as tight as the betting had expected it to be, and it took Thiem over an hour to finally edge it on a tie-break he had dominated before the Greek youngster came roaring back.
Eventually, Thiem sent down an ace to set up a set point which he took on his opponent's serve, but Tsitsipas responded in style with two early breaks as he dominated the second set.
Tsitsipas said: "I was playing with nerves, it's such a big event. I was a break up in the third set, but couldn't hold it, but I'm so relieved with this outstanding performance I gave on the court.
"It's phenomenal, unbelievable to have such an army behind me, they give me energy, and belief, and I just love that. I would like to thank everyone who supported me with a Greek flags and made me feel at home."