Croatia's Marin Cilic and Borna Coric are the latest players to join Novak Djokovic's eight-man tournament across the Balkans.
Played between June 13-July 5, the tournament will be held behind closed doors with Serbia's lockdown measures still in place.
"I thank Marin and Borna for joining the tour without asking for any financial compensation," said Djokovic.
The tournament will also feature Alexander Zverev, Dominic Thiem and Grigor Dimitrov, with the first leg set to take place in Djokovic's hometown of Belgrade on June 13 and 14.
Each event will feature two pools of four players with the winner progressing to a final.