Croatia's Borna Coric became the second player from Novak Djokovic's Adria Tour exhibition tournament to test positive for Covid-19, raising serious concerns for tennis governing bodies in their bid to restart the sport after a lengthy shutdown.
The professional circuit was halted in early March as nations closed borders and imposed lockdowns to contain the spread of the virus.
Last week the men's ATP and the WTA, which runs women's events, issued revised calendars for the resumption of the circuit from August while organisers of the US Open said the Grand Slam will be staged without fans as scheduled from Aug. 31 to Sept. 13 in New York.
Those plans, however, could need to be redrawn with Coric's positive test following that of Grigor Dimitrov, who said on Sunday he had contracted the novel coronavirus, highlighting the risks of athletes from different countries mixing without adhering to social distancing norms.
The ATP and the WTA did not respond to request for comments. Organisers of the Adria Tour could not reached for comment.
Bulgarian Dimitrov flew in to play in Djokovic's event in Belgrade and the second leg in the Croatian coastal resort of Zadar as did other top names like Dominic Thiem of Austria and Germany's Alexander Zverev.
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 under way.
Coric, ranked No.33 in the world, revealed on Twitter that he had tested positive for Covid-19.
"I want to make sure anyone who has been in contact with me during the last few days get tested," the 23-year-old said. "I am really sorry for any harm I might have caused. I'm feeling well and don't have any symptoms."
The tournament witnessed packed stands during the opening leg in Belgrade, players hugging at the net, posing for pictures together and attending press conferences together.
Djokovic organised nights out in Belgrade for the players and himself and pictures and videos of him dancing with the other participants at his event were posted on social media.
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 on Monday.
With international tennis suspended, Djokovic organised the Adria Tour as a charity event to be contested over four legs across the former Yugoslavia.
Djokovic's fitness coach Marco Panichi has also tested positive, Serbian daily Sportski Zurnal reported, quoting Croatia's health institute.
The men's world number one took the test on Monday after returning to Belgrade in the morning, Serbian media reported.
Russian Andrey Rublev, another player at the Adria Tour, said he tested negative for the virus.
"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," Australian player Nick Kyrgios said in a scathing social media post.
American tennis great Chris Evert also 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."
Austrian Thiem, the current men's number three, is currently playing in France in the Ultimate Tennis Showdown (UTS), which is co-owned by Serena Williams' coach Patrick Mouratoglou.
Organisers of the UTS, which also features men's top-10 players like Greek Stefanos Tsitsipas and Italian Matteo Berrettini, posted pictures on Friday of Thiem undergoing test after arriving at the Mouratoglou academy in Nice.