World number one Novak Djokovic said he is "so sorry" after becoming the latest tennis player to test positive for Covid-19.
Grigor Dimitrov, Borna Coric and Viktor Troicki all revealed they had coronavirus after playing at Djokovic's Adria Tour competition.
Djokovic, 33, played fellow Serb Troicki in the first event in Belgrade.
In a post on Twitter, Djokovic said it had been "too soon" to stage the tournament.
"I am so deeply sorry our tournament has caused harm," added Djokovic.
He said the tournament had been organised with "a pure heart", "good intentions" and a belief that they had "met all health protocols".