Australian Open chief Craig Tiley said he expected "decisions soon" on arrangements for the tournament after reports said it could be shifted from its usual January slot.
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.
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.
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.
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.
Rafael Nadal says he is determined to improve his poor record on indoor courts as he bids to win the ATP Finals for the first time in his illustrious career.
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.
Alexander Zverev produced a brilliant display on Saturday to end Rafael Nadal's hopes of a first Paris Masters title and set up a final against Daniil Medvedev.
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.
Rafael Nadal beat Australian Jordan Thompson 6-1, 7-6 (7/3) to set up a quarter-final match against Pablo Carreno Busta.
Rafael Nadal said he was proud of a "great achievement" after securing the 1,000th Tour-level victory of his illustrious career on Wednesday, becoming the fourth man to reach that mark with a comeback win over Feliciano Lopez in the Paris Masters second round.
Novak Djokovic has now spent 293 weeks as the world's best player, edging closer to Roger Federer's record of 310.
Novak Djokovic has been the top-ranked player at the end of a season five times: in 2011, 2012, 2014, 2015 and 2018.
Novak Djokovic said he hopes to play next year's Australian Open in a bid to chase Roger Federer and Rafael Nadal's record Grand Slam titles.