Grigor Dimitrov beat David Goffin 7-5, 4-6, 6-3 in a gripping title decider at the ATP Finals to become the first debutant to win the season finale since 1998.
In a surprise final match-up, the Bulgarian sixth seed held his nerve under intense pressure at london's O2 Arena to seal the biggest title of his career on his fifth match point.
He will climb to third in the rankings behind only Rafael Nadal and Roger Federer as he begins to deliver decisively on his rich promise as a youngster.
"It has been a tremendous two weeks for me," said the 26 -year-old. "It is such an honour to play here. This week has been one of the best I have had."
"I am lost for words," he added. "My team have been unbelievable, my family."
The Bulgarian came into the title decider unbeaten at the O2, with a healthy 4-1 head-to-head record against Goffin, including a 6-0, 6-2 win in the round-robin stage.
But seventh seed Goffin was buoyed by wins over world number one Nadal and and a gutsy effort against second seed Federer, coming back from a set down to beat the Swiss in the semi-finals.
Dimitrov has enjoyed the best season of his career, claiming his first Masters title in Cincinnati and winning two other titles aside from the ATP Finals as well as reaching the Australian Open semi-finals.
Nicknamed "Baby Fed" early in his career for the similarity of style in his game to the Swiss, Dimitrov has struggled to live up to the comparison, slipping to 17th in the rankings at the end of 2016.