Fourth seed Medvedev, 23, had several chances to break his Greek opponent in the second set, however it was Tsitsipas who finally seized the initiative as his rival made unforced errors.
The 21-year-old Greek, seeded fifth, secured a crucial break in the ninth game to take a 5-4 lead, and Medvedev’s efforts to save the game fell short as Tsitsipas saw out his maiden victory at the ATP Finals.
✅ First win at the Nitto ATP Finals ✅ First #ATPTour win over Daniil Medvedev
The other match of the Andre Agassi Group saw Swiss tennis icon Roger Federer sensationally lose to Austria’s Dominic Thiem with a twin score of 7-5, 7-5 on Sunday.
In the tournament’s curtain-raiser, Serbian tennis star Novak Djokovic had dispatched Italian debutant Matteo Berrettini 6-2, 6-1 to register his first win in the Bjorn Borg Group.
Spaniard Rafael Nadal, who has also been placed in the Andre Agassi group, will start his ATP Finals quest on Monday with a match against defending champion Alexander Zverev of Germany.
