Croatian tennis player Borna Coric has revealed he has contracted the coronavirus, becoming the second player to participate in the Adria Tour to return a positive test.
Coric's diagnosis follows that of Grigor Dimitrov, who said on Sunday he had contracted Covid-19.
Bulgarian Dimitrov also played in Novak Djokovic's exhibition event and the final in the Croatian coastal resort of Zadar was cancelled as a result.
Organisers of other sports looking to resume from the coronavirus shutdown will be watching developments with some trepidation as they try to find the safest way of getting competition back underway.
"I want to make sure anyone who has been in contact with me during the last few days get tested," the 23-year-old Coric wrote on Twitter. "I am really sorry for any harm I might have caused. I'm feeling well and don't have any symptoms."
With international tennis suspended due to the Covid-19 pandemic, world No 1 Djokovic organised the Adria Tour as a charity event to be contested over four legs across the region.
Djokovic's fitness coach Marco Panichi has also tested positive, Serbian daily Sportski Zurnal reported, quoting Croatia's health institute.
The tournament witnessed packed stands during the opening leg in Belgrade, players hugging at the net, posing for pictures together and dancing at clubs.
With both Serbia and Croatia easing lockdown measures weeks before the event, players were not obliged to observe social distancing rules in either country.
"It is recommended that anyone who has been in close contact for more than 10 minutes with the tennis player be advised to self-isolate for 14 days and to contact their doctor," the event organisers said in a statement.
Australian Nick Kyrgios asked on Twitter if Djokovic, Austrian Dominic Thiem and German Alexander Zverev had been tested following Dimitrov's result.
American tennis great Chris Evert has criticised the organisers for the lack of social-distancing, saying, "Total physical contact, no face masks, even the fans were without masks. I don't get it."