Daniil Medvedev claimed the biggest title of his career to become the last winner of the Nitto ATP Finals at London's O2 Arena.
The Russian went into the final against Dominic Thiem having not lost a match all week and he fought back from a set down to win 4-6 7-6 (2) 6-4.
In doing so, Medvedev became the first player in the 50-year history of the tournament to beat the world's three leading players at the same event.
The 23-year-old's triumph also meant London's tenure was bookended by Russian triumphs, with Nikolay Davydenko lifting the trophy in its first year in 2009.
Thiem said: "Of course I'm disappointed but, at the same time, I'm also proud of the performance of all the week.
"Daniil really deserved it. Amazing match. Congrats for another great year as well. I hope that we'll have many great matches to come. It was a pleasure today even though I lost."
Medvedev returned the praise to Thiem, saying: "What a match. One of my best victories. Against an amazing player, Dominic. Congrats already for what you achieved in your career. Your name is already in the history of tennis.
"You won a grand slam this year, you're playing unbelievable. I hope we're going to have many more matches to come on the big occasions like this."
Meanwhile, Australian Open organisers hope to finalise tournament dates "as soon as possible" amid speculation January's event could be moved to February or March.
Current Covid-19 quarantine restrictions in Australia would make holding the first grand slam of 2021 in its planned January 18-31 slot logistically difficult.
At present, players need to quarantine for two weeks on arrival in Australia while the state of Victoria will reportedly not allow them to enter before January 1, impacting warm-up tournaments and the players' ability to train and prepare.