Grigor Dimitrov beat David Goffin 7-5, 4-6, 6-3 to win the ATP Finals on Sunday, becoming the first debutant to triumph at the end-of-season finale since 1998.
The Bulgarian sixth seed took the first set with three breaks of serve but faltered in the second as his Belgian opponent levelled the match.
As the tension mounted in the decider, Dimitrov passed up four championship points but made the fifth one count, to win the biggest title of his career.
Dimitrov "lost for words" after ATP Finals win
"It has been a tremendous two weeks for me," he said. "It is such an honour to play here. This week has been one of the best I have had.
"David is such a tremendous guy, forget the tennis. He can hit the ball well also so congrats to him. He is one of the most improved players this past week and months. It was an unbelievable effort. I am very proud to play him in the final.
"I am lost for words. My team have been unbelievable, my family."
The Bulgarian came into the title decider unbeaten at London's O2 Arena, with a healthy 4-1 head-to-head record against Goffin, including a 6-0, 6-2 win in the round-robin stage.
But Goffin was buoyed by wins over world number one Rafael Nadal and and a gutsy effort against second seed Roger Federer, coming back from a set down to beat the Swiss in the semi-finals.
Both players struggled to hold serve at the start of the contest in front of a feisty packed house and the error count was high from both men.
After three consecutive breaks, the Belgian was the first to hold his serve, forging ahead 3-1 and the match then settled back into a more predictable pattern.
But as time ran out for Dimitrov, Goffin played a poor service game, going long on a forehand to concede another break of serve as the Bulgarian levelled the match at 4-4.
The errors continued to flow from a nervy-looking Goffin and Dimitrov finally secured the decisive break on his fifth set point when the Belgian thumped a forehand into the net.
Goffin, 26, landed just 42 percent of first serves in the first set and made 20 unforced errors, struggling to find the bite and consistency he had enjoyed in beating Federer.
In the second set, neither player earned a break point until the sixth game, when Goffin saved himself with a dramatic backhand crosscourt shot that caught the sideline.
With the pendulum suddenly swinging, Goffin took advantage of two double faults from the racquet of Dimitrov, smashing a forehand winner past him to seal the break in the next game.
Infused with renewed belief, the Belgian held his nerve to clinch the set 6-4.
Despite a sprinkling of break points, a tight third set went with serve until the sixth game, when Dimitrov earned a break to lead 4-2 as Goffin went wide with a backhand.
As the tension mounted, Goffin bravely saved three championship points on his own serve but Dimitrov held his nerve to seal the match on his after two-and-a-half hours, at the fifth time of asking.
Dimitrov emulates Spain's Corretja
Dimitrov, 26, has enjoyed the best season of his career, claiming his first Masters title in Cincinnati and winning two other titles as well as reaching the Australian Open semi-finals.
He is guaranteed to finish the season third in the rankings.
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 the 2016 season.
The final that nobody expected came just a year after Novak Djokovic and Andy Murray battled it out in London for the number one slot.
But both players have been absent from the Tour for months and with Nadal also forced to pull out of the ATP Finals after his round-robin opener, the end-of-season finale has had an unusual flavour.
Sunday's final was the first time in the tournament's history that two players who qualified for the first time faced off in the title decider.
The last player to win the trophy in their first year as a qualifier was Alex Corretja in 1998.