There was little to choose between the two players in a high-quality first set. Neither player managed a break of serve, but it was Thiem who emerged the stronger in the tiebreak and sealed the first set with an ace.
Tsitsipas produced an emphatic response to losing the opening set, breaking Thiem twice to convincingly take the second 6-2 and force the final into a decider.
The two players traded breaks at the start of the decider, but neither could find another as a tight and fiercely contested final was decided by another tiebreak.
This time it was Tsitsipas who managed to hold his nerve when it mattered most to take a thrilling victory and seal the biggest title of his career so far.
Tennis TV showed Tsitsipas' moment of victory:
Tsitsipas showed few signs of nerves in the early exchanges, holding to love in the opening game of the match and earning the final's first break point in Thiem's second service game.
Yet the Austrian was quick to close the opportunity down, producing some powerful groundstrokes and clinical serving to hold for 2-2.
Thiem was having to work harder than his opponent to hold serve but managed to gain his first break points at 3-3. Some sloppy play and poor forehands gifted Thiem two break points, but Tsitsipas responded with a brilliant serve and volley off a second serve and then clinched the hold with a big forehand.
Thiem survived more break points with some brave serving in the next game, and it was little surprise to see the first set go all the way to the tiebreak.
A missed forehand from Tsitsipas gave Thiem the early advantage and saw him move 3-0 up. The Greek star did manage to pull it back to 5-5, but a wild backhand followed by a precise serve from Thiem down the middle sealed the opener for the Austrian in one hour and five minutes.
Tsitsipas roared straight back in the second set, earning three break points in the opening game. Thiem saved the first but sent a forehand wide to hand his opponent the early advantage.
Journalist Jose Morgado noted how it was an important moment in the match:
The Greek star held comfortably and then broke again to take a 3-0 lead, sealing his second break in a row with a brilliant forehand down the line.
Freelance writer Ben Rothenberg highlighted his dominance:
Thiem finally got on board at 4-1 but could do nothing to prevent the sixth seed from seeing out the set 6-2 in just 28 minutes to send the final into a decider.
Tsitsipas continued to pile on the pressure at the start of the third. Some great court coverage and ferocious backhands forced two more break points, but he could not convert as Thiem dug deep to secure the hold.
Yet the pressure from Tsitsipas was relentless, and he forced two more break points in Thiem's next service game. This time, the Austrian could not cope and a backhand into the net gave the 21-year-old the break and a 2-1 lead.
Thiem responded with some brilliant defensive tennis to break back with Tsitsipas serving at 3-2. A wayward backhand eventually gifted the fifth seed the break and was followed by a hold to love.
Both players raised their level toward the end of the match. But neither could find another break, and a tiebreak was needed to decide the winner.
Thiem had his chances, but a backhand into the net at 5-4 handed Tsitsipas match point, and the 21-year-old made no mistake to become the youngest ATP Finals debutant to win the title since John McInroe in 1978, per Stuart Fraser at The Times.