Borna Coric confirmed he has coronavirus just a day after the Adria Tour final was cancelled due to Grigor Dimitrov's positive test.
Sunday's match between Novak Djokovic and Andrey Rublev was called off after Dimitrov announced he had tested positive for COVID-19.
The world number 19 had played in the Serbian and Croatian legs of the exhibition tournament and lost in straight sets to Coric in Zadar on Saturday before returning home after feeling unwell.
In a Twitter post on Monday, Coric said: "Hi everyone, I wanted to inform you all that I tested positive for COVID-19.
"I want to make sure anyone who has been in contact with me during the last few days gets tested!
"I am really sorry for any harm I might have caused! I'm feeling well and don't have any symptoms.
"Please stay safe and healthy! Lots of love to all!"
Australian Nick Kyrgios responded to Coric's Twitter post to slam the decision to hold the event.
"Boneheaded decision to go ahead with the 'exhibition' speedy recovery fellas, but that's what happens when you disregard all protocols. This IS NOT A JOKE," he wrote.
The tournament, set up by Djokovic as a means of helping top players return to fitness after the ATP Tour was suspended due to the pandemic, attracted thousands of fans in Serbia and Croatia, where lockdown measures were being eased.
Professional tennis has been largely at a standstill since March, with the French Open pushed back to September, Wimbledon cancelled and the US Open set to be staged without fans and under strict health and safety protocols from August 31.