Dubbed "Novaxx Djocovid" for his anti-vaccination stance and positive COVID-19 test, the world No.1 became the fourth player from his own charity event in Croatia to fall victim to the virus.

Savaged on social media, Djokovic issued an apology after footage emerged of the Serbian superstar partying with fellow players in a nightclub.

Australian Nick Kyrgios was among those to let rip at Djokovic for not following social distancing protocols.

"Prayers up to all the players that have contracted Covid- 19," Kyrgios posted while retweeting a video of the players dancing shirtless in Belgrade.

"Don't @ me for anything I've done that has been 'irresponsible' or classified as 'stupidity' - this takes the cake."

Now self-isolating along with his wife Jelena, who also tested positive, Djokovic said: "I am so deeply sorry our tournament has caused harm.

"We believed the tournament met all health protocols and the health of our region seemed in good conditions to finally unite people for philanthropic reasons.

"We were wrong and it was too soon. I can't express enough how sorry I am for this and every case of infection."

But his apology is unlikely to cut the mustard, with fellow Player Council representative Bruno Soares also condemning his conduct.

"I sum it up as a horror show," Soares told Brazilian news outlet GloboEsporte.

"Enormous irresponsibility and huge immaturity. They were totally careless, and it's difficult for me to find the words."

When the US Open this month announced its plans to protect players from COVID-19 by limiting the size of entourages and restricting players' movements, Djokovic himself was an outspoken critic and calling the plan "extreme" and questioned whether he would play.

But now Djokovic's fitness coach Marco Panichi along with fellow Serb Viktor Troicki, Croat Borna Coric and Bulgarian Grigor Dimitrov plus his coach Christian Groh have all tested positive after the controversial Adria Tour event in the Balkans.

While the US Open is still two more than two months from its scheduled start, the prospect of Djokovic having more than one person from his own entourage at Flushing Meadows would seem remote.

Even the New York grand slam going ahead has been questioned.

"Yikes this is not good and it's a pattern. Hope Novak will be ok of course! What now, US Open? Roland Garros? We have a lot of work to do," tweeted 18-times grand slam singles champion Martina Navratilova.

While unimpressed by Djokovic's actions, former world No.1 and 2012 Flushing Meadows winner Andy Murray said he didn't expect the 2020 US Open to be scrapped.

"Obviously it's not surprising how many players have tested positive when you see the scenes and the images and the videos from the tournament and the players party with no social distancing in place," Murray said.

"I've seen some people say this puts the US Open in doubt but the measures and the protocols they have in place are completely different to what was going on in Serbia and Croatia.

"For a start, there will be no fans and the players will now know we can all be affected by this, it doesn't matter who you are, we need to respect the rules."

Australian Associated Press