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.
Novak Djokovic out-battled Alexander Zverev to qualify for the last four at the ATP Finals on Friday, staying on track to match Roger Federer's record of six titles at the end-of-season event.
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.
Daniil Medvedev shocked Novak Djokovic in straight sets to book a place in the last four at the ATP Finals on Wednesday
Novak Djokovic is playing in the ATP Finals in London at the end of a coronavirus-truncated season but the new campaign is due to start in just seven weeks' time.
Novak Djokovic defeated Diego Schwartzman 6-3, 6-2 at the ATP Finals in London.
Rafael Nadal barely broke sweat as he outclassed the Russian debutant Andrey Rublev 6-3, 6-4 at the empty O2 Arena.
Roger Federer may have faced a stern challenge from the likes of Rafael Nadal and Novak Djokovic, but his toughest opponent on court was finally unveiled in a hilarious video by Rajasthan Royals.
Novak Djokovic attempted to replicate Cristiano Ronaldo's celebration after scoring a goal, with the Portuguese providing his opinion on this attempt.
The top eight available players in the men's rankings -- with Roger Federer absent -- will compete at the end-of-season tournament, which starts on Sunday behind closed doors.
Nick Kyrgios said he had always been emotional and found it tough spending so long away from his Canberra home and family while on tour, admitting there were times when he was "seriously depressed".
Novak Djokovic, 33, was confirmed in the position after Rafael Nadal, the only man who could deny him top spot, opted not to play in the Sofia event next week.
Novak Djokovic has now spent 293 weeks as the world's best player, edging closer to Roger Federer's record of 310.
Novak Djokovic's daughter Tara decided to style him and the results were quite different.
Having won two rounds in the Austrian capital, Novak Djokovic is virtually guaranteed equalling Pete Sampras's record of securing the year-ending top ranking for the sixth time.
Italian Lorenzo Sonego, ranked 42 and who had initially lost in qualifying last weekend, is the first lucky loser ever to beat Novak Djokovic.