Alexander Zverev and Nick Kyrgios were slated to join Dominic Thiem at an exhibition tournament from July 13-19 in Berlin.
Alexander Zverev was heavily criticised after viral footage emerged of him partying in a busy bar despite pledging to "follow self-isolating guidelines" after playing in Novak Djokovic's coronavirus-hit Adria Tour.
Nick Kyrgios was involved in a Twitter battle with Boris Becker after the Australian took issue with fellow-tennis player Alexander Zverev.
After testing negative for coronavirus, Alexander Zverev had released a statement vowing to self-isolate, only to be spotted at a busy bar, apparently over the weekend, in footage that went viral.
Dominic Thiem took part in the Belgrade leg of the exhibition event organised by Novak Djokovic where social distancing was minimal and matches were played in front of thousands of fans.
Nick Kyrgios, Dominic Thiem and Alexander Zverev have been signed up to take part in Berlin.
Novak Djokovic addressed 4,000 fans packed into the Novak Tennis Centre on the banks of the Danube in the first leg of his Balkans charity tennis tournament.
The US Open main draw is due to get underway in New York on August 31.
Top-level international tennis returns this weekend in Belgrade with a charity tournament hosted by world number one Novak Djokovic and where fans will be welcome.
The organisers said an audited attendance of 51,954 beat the 42,517 who watched Roger Federer play Alexander Zverev of Germany in Mexico City last November.
Australian Open final: Novak Djokovic is 6-4 in their career head-to-heads, but Dominic Thiem has won four of the last five.
Dominic Thiem came from a set down to outlast Alexander Zverev and make his first Australian Open final on Friday.
Alexander Zverev stormed into his first Grand Slam semi-final, rallying from a set down to shatter the dreams of Stan Wawrinka at Australian Open.
Stefanos Tsitsipas was in a different league to Alexander Zverev, who only began hitting balls a week ago after an off-season curtailed by a lucrative exhibition tour.
Dominic Thiem saw off the big-serving German 7-5, 6-3 after Greece's Stefanos Tsitsipas, making his ATP Finals debut, had earlier beaten Roger Federer.
Rafael Nadal exited the ATP Finals but ended his season as the world number one and with a chance to lead Spain to Davis Cup glory.
Rafael Nadal has never won the end-of-season tournament and had a miserable start at the O2 Arena, being outplayed by Germany's Alexander Zverev.