Novak Djokovic has announced Croatia's Marin Cilic and Borna Coric as the latest players to join an eight-man tournament to tour the Balkans.
Austria's Dominic Thiem, Bulgaria's Grigor Dimitrov and Germany's Alexander Zverev - all current or former top-three players - will also take part.
The first leg will take place in Djokovic's hometown of Belgrade on 13 and 14 June.
"I'll do everything in my power to be a good host," said the world number one.
The tournament could be behind closed doors with Serbia's coronavirus containment measures still banning large-scale gatherings.