Medvedev, the No. 4 player in the ATP rankings, claimed the title with a 4-6, 7-6 (2), 6-4 victory over Thiem on Sunday. He beat Djokovic -- the world's top player -- in straight sets on Wednesday in the group stage, then Nadal -- the No. 2 player -- in three sets in Saturday's semifinal to clinch his match-up against the third-ranked Thiem.
Medvedev is the first player in history to beat the top three men's players in the ATP Finals, which is the season finale for the men's tour. He fired 12 aces and 37 winners in the two-hour, 42-minute final. Thiem hit six aces and 29 winners in the loss.
"What a match," Medvedev told reporters. "[It was] one of my best victories of my career -- two hours [and] 42 [minutes], three sets against an amazing player.
"Dominic, congratulations already for what you achieved in your career. Your name is already in the history of tennis books. It is amazing. You won a Grand Slam this year. You are playing unbelievable. I hope we are going to have many more matches to come on big occasions like this."
Medvedev is just the fourth player in history to beat the top three men's players in one tournament. He also defeated No. 9 Diego Schwartzman and No. 7 Alexander Zverev in the group stage.