World number one Novak Djokovic was widely condemned on Wednesday for hosting a tennis exhibition where he was one of four players to test positive for the coronavirus, a lapse that sent shudders through a sport struggling to get back on its feet.

The Serbian star said he was "deeply sorry" in an unstinting apology for the now-cancelled Adria Tour, where social distancing was minimal and matches were played in front of thousands of fans.

However, criticism was swift and heavy, with many voicing concerns over attempts to restart professional tournaments in August, including the US Open Grand Slam is which is scheduled to begin on August 31.

Djokovic, Grigor Dimitrov, Borna Coric and Viktor Troicki all tested positive after taking part in the Adria Tour, where players embraced across the net, played basketball and even danced in a nightclub.

"I am so deeply sorry our tournament has caused harm," Djokovic, 33, said in a statement. His wife Jelena also tested positive after attending the "philanthropic" tour in the once war-torn Balkans.

As the mocking hashtag #Djokovid circulated online, Australia's Nick Kyrgios, so often in the crosshairs for his own on-court indiscretions, said the incident was pure "stupidity".

"Don't @ me for anything I've done that has been 'irresponsible' or classified as 'stupidity' -- this takes the cake," tweeted the world number 40.

Britain's Andy Murray, a three-time Grand Slam winner who has known Djokovic since their junior days, said: "I don't think it has been a great look for tennis."

"In hindsight, it's not something that should have gone ahead," Murray told reporters.

"It's not surprising how many people have tested positive after seeing some of the images of the players' party and the kids' day. There was no social distancing in place.

"Some people have said maybe this has put the US Open in doubt –- which it may well do. But the measures and the protocols they have in place at the USTA (United States Tennis Association) are different to Serbia and Croatia. No fans for a start."

- PR disaster -
Djokovic is unbeaten this year, a run that includes winning his 17th Grand Slam title at the Australian Open, but COVID-19 has been a public relations disaster for the eccentric Serb.

Even before the Adria Tour, he was criticised for breaking lockdown rules to train in Spain, and he then raised eyebrows by insisting he wouldn't be prepared to vaccinate against the coronavirus.

Djokovic also described limits on players' entourages at the US Open as "extreme" and "impossible", again putting him at odds with much of public opinion.

His latest misstep has caused some to question his presidency of the ATP (Association of Tennis Professionals, or men's tour) Player Council, which advises the ATP board.

Tennis has been particularly hard-hit by the coronavirus pandemic and the resulting travel restrictions because it is a global circuit with players from all over the world.

"I think there's a lot of his peer group who are scratching their heads," celebrated coach Paul Annacone told Tennis.com.

"I was totally anxiety-ridden and very disappointed because the restart, or the reimagining of how we can start (tennis) is just about eight weeks away. And with all these opportunities to try to start in a progression, to me, it felt like they skipped about 15 steps."

Martina Navratilova, who won 18 Grand Slam singles titles, tweeted: "Yikes... this is not good and it's a pattern... What now, US Open? Roland Garros? We have a lot of work to do."

Brazil's Bruno Soares, a doubles player who sits on the Player Council, called the Adria Tour a "horror show", while ATP chairman Andrea Gaudenzi said it was a lesson for other tournaments.

"It's a little bit like when you tell your kids when they try to learn to ride the bike to wear the helmet," Gaudenzi said. "It's 'no, no, no'."

"And they ride the bike, they fall, and then they wear the helmet."