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 organizers 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 after three players who had featured in the Adria Tour event tested positive for the coronavirus. On Sunday Grigor Dimitrov said he had contracted the novel coronavirus.

While the players did not break any Serbian or Croatian government protocols, as neither country required the athletes to maintain any social distancing, the fact that three players and some support staff caught the virus highlights the risks of athletes from different countries being in close proximity to each other.

"I want to make sure anyone who has been in contact with me during the last few days gets tested," Coric, ranked No.33 in the world, said on Twitter. "I am really sorry for any harm I might have caused. I'm feeling well and don't have any symptoms."

Troicki played in the first leg staged at Djokovic's tennis complex in central Belgrade from June 13-14, before Dimitrov and Coric tested positive at the second leg in Zadar held from June 20-21.

Troicki, 34, entered Janko Tipsarevic's Eastern European Championship tournament which started in Belgrade last Monday and pulled out after learning that he and his wife tested positive.

"My wife took the test on Friday and I took it on Sunday after she tested positive," Troicki told Serbia's Telegraf website. "Our daughter tested negative."

The ATP wished players and staff who tested positive at the event a speedy recovery and urged "strict adherence to responsible social distancing" to limit the spread of the virus.

The governing body added that it will implement a variety of precautions and protocols that would be adjusted as and when dictated by the latest medical information to ensure safety at events when the tour resumes on Aug. 14.

The WTA did not respond to a request for comment. Organizers of the Adria Tour could not be reached for comment.

Bulgarian Dimitrov flew in to play in Djokovic's event in Belgrade and the second leg, as did other top names like Dominic Thiem of Austria and Germany's Alexander Zverev.

Organizers 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.

No social distancing

The tournament witnessed packed stands during the opening leg in Belgrade, players hugging at the net, playing basketball, posing for pictures and attending press conferences together.

Djokovic organized nights out in Belgrade for the players 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 organizers said in a statement on Monday.

With international tennis suspended, Djokovic organized the Adria Tour as a charity event to be contested over four legs across the former Yugoslavia.

Djokovic's fitness coach Marco Panichi 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 and his test results will be announced on Tuesday, after which he is set to address the public, Serbian media reported.

Zverev, Croatia's Marin Cilic and Russian Andrey Rublev - who all played at the Adria Tour - said they tested negative for the virus but will follow self-isolation guidelines.

"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 criticized the organizers for the lack of social-distancing, saying "... total physical contact, no face masks, even the fans were without masks.. I don't get it."

Thiem, the 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 a test after arriving at the Mouratoglou academy in Nice.