In an event that had previously been dominated by Novak Djokovic, Medvedev was the fifth different first-time winner since the last of the Serbian’s victories at the end-of-season finale.
For much of the opening set and a half, it looked in danger of being a damp squib as Thiem comfortably held the upper hand.
But his rival turned it around from 2-0 down in the tiebreak to win seven unanswered points and then dominated the rest of the match for a 4-6, 7-6, 6-4 victory.
It was an apt winner for London’s farewell as host to these finals, with Medvedev only the second Russian winner in the event’s history, the first being Nikolay Davydenko when it made its debut in the capital in 2009.
“I always said it would be amazing if the first champion was Russian and the last champion was last Russian," he said. “So a big thank you to Nikolay for being an inspiration.”
Medvedev and Thiem had comfortably been the best two players this week, and it could be an event to act as a changing of the guard in tennis with Thiem beating Djokovic in three tight sets in the semi-finals, and Medvedev also being taken the distance in his previous encounter by Rafael Nadal.
There were echoes of Djokovic in this final. In 2007, he had lost all of his round-robin matches, as did Medvedev on his event debut last year. And, like Djokovic had done in 2008, 12 years later the Russian did the same.
The enormity of the occasion appeared to get to Medvedev in his fifth career meeting with Thiem as the Austrian used his slice to good effect to negate the ferocity of his opponent’s forehand.
There appeared a danger of London’s farewell as host – Turin will replace it next year – fizzling out on the 50th anniversary of the event.
But an out-of-sorts Medvedev found his rhythm against the US Open champion as the second set dragged on, not surprising for a player with a 50% win ratio when losing the first set, as had been the case against Nadal in the previous round.
Both men found their serve under pressure but held to take it to a second-set tiebreak. Thiem looked in command when he won the opening two points before Medvedev won seven unanswered points.
It changed the tide of the game, Thiem for so long the aggressor suddenly on the defensive and, when a Medvedev volley, sealed the only break for a 3-2 lead, Thiem’s fight was partially extinguished. From that point, Medvedev said in command.
For Thiem, it was a second straight final defeat at the ATP Finals. Afterwards, he said: “I’m disappointed but at the same time I’m also proud of the performance of all the week. Daniil really deserved it. An amazing performance. “