The 29-year-old Bulgarian tennis player had been competing in the Adria Tour exhibition tournament in Croatia until he withdrew as he fell ill during the weekend.
Grigor Dimitrov said he has tested positive for coronavirus after pulling out of an exhibition tournament which also featured world number one Novak Djokovic on Sunday.
"I want to reach out and let my fans and friends know that I tested positive back in Monaco for Covid-19," world number 19 Dimitrov wrote on Instagram.
"I am so sorry for any harm I might have caused. I am back home now and recovering."
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Hi Everyone-I want to reach out and let my fans and friends know that I tested positive back in Monaco for Covid-19. I want to make sure anyone who has been in contact with me during these past days gets tested and takes the necessary precautions. I am so sorry for any harm I might have caused. I am back home now and recovering. Thanks for your support and please stay safe and healthy. GD
Last weekend, Dimitrov, 29, took part in the Adria Tour event in Belgrade with Djokovic, world number three Dominic Thiem and Alexander Zverev, ranked seven.
He also played in the second leg of the Balkans tournament in Zadar on Croatia's Adriatic coast on Saturday.
However, he withdrew after losing to Borna Coric, complaining of feeling unwell.
Later on Sunday, the final between Djokovic and Russia's Andrey Rublev was immediately cancelled as a precaution.
"This is the best possible decision. We have to take care about security of all involved," former Wimbledon champion Goran Ivanisevic, the tournament director of the Zadar event, said in remarks quoted by local media.
"Those who were in contact with Grigor will be tested. I tested by chance three days ago and I'm negative."
Zverev and former US Open winner Marin Cilic were also in the Croatia line-up.
"I want to make sure anyone who has been in contact with me during these past days gets tested and takes the necessary precautions," added Dimitrov.
Djokovic's brother Djordje, who is the overall tournament director, told Sportske Novosti daily paper online edition: "Novak? He took this news very hard. We undertook all the measures prescribed by the governments of Serbia and Croatia."
Bulgarian Dimitrov is a former world number three who was the ATP Tour Finals champion in 2017, one of his eight career titles.
Social distancing measures questioned
The Adria Tour, organised to fill the gap in the virus-hit tennis calendar which has been on ice since mid-March, played out to a daily crowd of 4,000 fans at Djokovic's tennis centre on the banks of the Danube in Belgrade last week.
Dimitrov, Djokovic and Thiem, as well as the other players, were then seen partying at a packed Belgrade nightspot.
Thiem has since travelled to the south of France to play in another exhibition tournament, the Ultimate Tennis Showdown in Nice.
On Sunday, tournament chief Patrick Mouratoglou, the coach of Serena Williams, tweeted a picture of Thiem undergoing a coronavirus test at the event being staged at his tennis academy.
The Adria Tour had already suffered an embarrassing setback when the planned Montenegro leg of the four-nation tournament was cancelled over coronavirus protocol rules.
Montenegro was due to be the third stop on June 27 and 28 after Croatia and before the conclusion in Bosnia.
But organisers said the visit to Montenegro had to be called off when it became apparent Serbia did not match strict health requirements.
Meanwhile, asked to comment on social distancing measures during the Belgrade leg, Djokovic said both Serbia and the region had been relatively successful in containing the virus.
"Of course you can criticise, you can also say this is dangerous or not, but it's not up to me to make the calls what is health-wise right or wrong," the 17-time Grand Slam winner told reporters, stressing he was acting in line with recommendations of the Serbian government.
Last week, the ATP and WTA said they were restarting their tours in August.
However, the US Open in New York will be played behind closed doors and under strict health protocols which Djokovic has described as "extreme" and "impossible".
The drama surrounding Dimitrov's test brought sympathy but also questions from fellow players.
"While watching images from the Adria Tour, I kind of felt that something was really wrong. Am I the only one? Is it a consequence of it? Get better soon Grigor," tweeted French player Alize Cornet.
Noah Rubin of the US added: "It's just not okay. Reckless and disappointing that they thought tennis deserved the risk."