Daniil Medvedev came back from a set down to beat Dominic Thiem 4-6, 7-6 (7/2), 6-4 and win the ATP Finals title.
Rafael Nadal, who has never won the elite event in his illustrious career, faltered when serving for the match in the second set and was beaten 3-6, 7-6 (7/4), 6-3.
Austria's Dominic Thiem, who beat Stefanos Tsitsipas and Rafael Nadal earlier in the week to win Group London 2020, was well below his best at London's empty O2 Arena in a 6-2, 7-5 defeat.
Daniil Medvedev shocked Novak Djokovic in straight sets to book a place in the last four at the ATP Finals on Wednesday
Former junior player Olga Sharypova said in a social media post last month that the German world number seven had tried to "strangle" her while at last year's US Open.
Third seed Daniil Medvedev claimed his first Paris Masters title at Bercy Arena on Sunday by battling back from a set down to beat Germany's Alexander Zverev.
Third seed Daniil Medvedev battled past Milos Raonic on Saturday to reach his first final of the season at the Paris Masters, where he will face either Rafael Nadal or Alexander Zverev.
Dominic Thiem will face Alexander Zverev in the US Open final after crushing Russia's Daniil Medvedev in straight sets in the semis.
Daniil Medvedev defeated Russian compatriot Andrey Rublev in straight sets to reach the semi-finals of the US Open on Wednesday.
Australian Open: Stan Wawrinka came from behind to beat World No. 4 Daniil Medvedev 6-2, 2-6, 4-6, 7-6 (7/2), 6-2 in five intense sets.
World number four Daniil Medvedev eased past Alexei Popyrin in straight sets on Saturday to set up a last-16 clash with former Australian Open champion Stan Wawrinka.
Daniil Medvedev overcame a nosebleed and stiff resistance from Spanish qualifier Pedro Martinez to enter Australian Open third round.
Defending champion Novak Djokovic insisted there are no clear favourites for the next week's Australian Open despite his ominous start to the season.
Novak Djokovic battled past fifth-ranked Daniil Medvedev 6-1, 5-7, 6-4 to send Serbia into the inaugural ATP Cup final.