Grigor Dimitrov, Borna Coric and Viktor Troicki also returned positive tests after playing in the Balkan tournament where players were seen hugging at the net, playing basketball, posing for pictures and partying like they did in pre-COVID-19 days.

Murray said he hoped the players and their family members who tested positive recovered soon and added that the tournament should not have gone ahead in hindsight.

"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 on Tuesday.

"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."

The players, however, did not break any government protocols in Serbia or Croatia with both countries easing lockdown measures weeks before the event.

On Tuesday Murray enjoyed a winning return to action as he beat Liam Broady in the 'Battle of the Brits' charity tournament at the national tennis center in west London.

The twice Wimbledon champion and former world number one eased to a 6-2 6-2 victory over the British number six in his first competitive match since the Davis Cup Finals in November.

The week-long event, organized by Murray's brother Jamie while professional tennis remains suspended due to the COVID-19 pandemic, is being played behind closed doors with strict health protocols in place.

While there were chair umpires, players picked up their own balls and towels and they did not change ends. The event is raising money for Britain's National Health Service.

There were also wins in London for British number one Dan Evans and Kyle Edmund on the opening day.

Murray will face Edmund in his next round-robin match.

"It was OK, I served well throughout the match. Didn't hit the ball that well from the back of the court, wasn't timing the ball well," he said.

"But it was okay. For a first match in seven months and not been practicing much, so it was alright."

"Last time I picked up my own balls was when I was about 17. Although we do it all the time in practice."

The ATP Tour was halted in early March as nations closed borders and imposed lockdowns to contain the spread of the novel coronavirus. Last week the ATP and the women's WTA issued revised calendars for the resumption of the circuit from August.