"The phenomenal news I want to announce today is that Sasa Zverev, Alexander Zverev, will come to Belgrade," Djokovic told a press conference in the Serbian capital on Monday. 
The German will join world number three Dominic Thiem and Grigor Dimitrov, ranked 19, in Belgrade on 13-14 June for the first stop of the Adria Tour. 
"Three of the world's greatest tennis stars. I'm really very grateful to them," said Djokovic.
Whether spectators will be allowed remains uncertain. 
"I hope some rules will be changed by June 13 and maybe we'll have the opportunity to have the audience as well," the world number one said. 
"Of course, this is still uncertain, like many other things related to this situation caused by the coronavirus."

The Adria Tour, which Djokovic announced on his 33rd birthday last week, will run from June 13 to July 5 in Serbia, Croatia, Montenegro and Bosnia on clay courts.

The money raised from the tour will be donated to various charities.

"It's humanitarian. That is to say, absolutely, all the income, whether it comes from sponsorship contracts, national and international audiovisual rights, everything will go to humanitarian organisations," he told the press.

Asked about his own shape, Djokovic said he had been training throughout his period of confinement in Spain, where he was staying with his family in a home in Marbella.

"I had a tennis court so I could train every day," he said.

The ATP and WTA Tours have been suspended since March and will not resume until at least the end of July, while Roland Garros was postponed to September and Wimbledon was cancelled for the first time since World War II.

Balkan countries have coped with the COVID-19 pandemic with relative success, with some 20,000 people infected and around 660 deaths in a region of some 22 million people.