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.
Dominic Thiem recovered from squandering four match points in a dramatic second-set tie-break to beat Novak Djokovic in the last four at the ATP Finals on Saturday.
Several major American sports have a policy setting out a procedure to be followed in such situations, and Novak Djokovic believes tennis should go down the same route.
Rafael Nadal won his opening round-robin match against Andrey Rublev but defeat against Dominic Thiem meant he faced a shootout against 2019 champion Tsitsipas to reach the semi-finals.
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.
Rafael Nadal insisted he can still win the ATP Finals for the first time despite a damaging 7-6 (9/7), 7-6 (7/4) defeat against Austria's Dominic Thiem on Tuesday.
Dominic Thiem held his nerve to claim an impressive 7-6 (9/7), 7-6 (7/4) win against Rafael Nadal at the ATP Finals.
Rafael Nadal barely broke sweat as he outclassed the Russian debutant Andrey Rublev 6-3, 6-4 at the empty O2 Arena.
Novak Djokovic has now spent 293 weeks as the world's best player, edging closer to Roger Federer's record of 310.
Italian Lorenzo Sonego, ranked 42 and who had initially lost in qualifying last weekend, is the first lucky loser ever to beat Novak Djokovic.
Novak Djokovic keeps his world no.1 ranking despite losing to Rafael Nadal in the French Open final.
Dominic Thiem knocked out of French Open after losing 6-7, 7-5, 7-6, 6-7, 2-6 to Argentna's Daniel Schwartzman in the quarter-finals.
Dominic Thiem survived a massive scare to reach the French Open quarters, but despite being stretched to his limit, the World No.3 still had time to showcase his mindblowing football skills.
Third seed Dominic Thiem survived a huge scare to reach the Roland Garros quarter-finals for a fifth straight year on Sunday
French Open: Rafael Nadal, the 12-time champion, faces world number 213 Sebastian Korda while US Open champion Dominic Thiem meets Hugo Gaston, ranked at 239.
World No.1 Novak Djokovic on Saturday demanded that the officials be replaced by machines.
French Open 2020: Rafael Nadal booked a last-16 berth, and is yet to drop a set in Paris.