Andy Murray's return to the Australian Open is in doubt after the Scotsman tested positive for COVID-19 just days before he was due to fly to Australia via a charter flight.

The three-time grand slam champion and five-time Australian Open finalist is isolating at home near London, the tournament said in a statement.

"The AO fans love Andy, and we know how much he loves competing here in Melbourne and how hard he'd worked for this opportunity."

Murray has reportedly not given up on making the trip and had earlier backed out of the Delray Beach Open in Florida to "minimise the risks" of contracting the virus through international travel.

However, that could prove difficult with players and officials required to arrive in Australia during a 36-hour window from Thursday and needing to serve a mandatory isolation of 14 days.

Murray has been given a wildcard entry into the first grand slam tournament of 2021.

Murray is not the only player to suffer a COVID blow with women's world number 16 Madison Keys pulling out after testing positive for COVID-19.

"I'm very disappointed to not be able to play in the coming weeks after training hard in the off-season and knowing Tennis Australia and the tours did so much to make these events happen," Keys wrote on Twitter, while also announcing her positive test.

Sandgren allowed to come to Australia despite positive test

While Keys is out, Murray could potentially compete in Australia, after American player Tennys Sandgren was given special clearance to board a charter flight from Los Angeles to Melbourne despite testing positive for COVID-19 in November and again on Monday.

Sandgren received an exemption after Australian health officials assessed his case history.

Sandgren tweeted he tested "COVID positive" on Monday, after originally testing positive in late November, and was initially barred from the flight containing international players out of Los Angeles.

But the world number 50 said he was then allowed to join the delayed flight, calling Australian Open tournament director Craig Tiley a "wizard".

Sandgren, a two-time Australian Open quarter-finalist, tweeted he was not contagious and had "totally recovered".

"My two tests were less than eight weeks apart. I was sick in November, totally healthy now," his tweet read.

COVID-19 Quarantine Victoria released a statement this afternoon saying it had reviewed Sandgren's positive test result "as per standard processes" and he was cleared to travel to Australia.

It said the evidence in Sandgren's case "suggested he had recovered from a previous COVID-19 infection and is still shedding viral particles".

"For people who have previously tested positive and have since recovered, it is common to shed viral fragments for some time — which can trigger another positive result," the statement read.

"Any person who returns a positive test result has their medical and case history reviewed by a team of public health experts.

"Only those who are determined to be recovered and no longer infectious will be allowed to travel to Australia."

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Victorian Police and Emergency Services Minister Lisa Neville supported the decision to allow Sandgren to board the flight to Australia.

"Tennys Sandgren's positive result was reviewed by health experts and determined to be viral shedding from a previous infection, so was given the all clear to fly," Ms Neville tweeted.

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Tennis Australia said it was following Victorian Government guidelines on players being allowed to fly to Melbourne.

"Any recovered case must go through this process in order to have an opportunity to travel here for the Australian Open," Tennis Australia said in a statement.

"No one can travel without either proof of a negative test or this special clearance from authorities confirming they are not infectious.

"Upon arrival all players are immediately placed in a secure quarantine environment for 14 days under the authority of COVID Quarantine Victoria and will undergo a more rigorous testing schedule than most returning travellers."

The Australian Open will start at Melbourne Park on February 8 and run until February 21.

The first charter flight carrying players contesting the Australian Open arrived in Melbourne this evening.

Earlier this week, three hotels were added to Victoria's hotel quarantine program to accommodate the arrivals.

These are the Grand Hyatt in Melbourne, the View on St Kilda Road, and the Pullman hotel in Albert Park.

Players and their support people will be allowed to leave their hotel rooms for up to five hours per day to complete training.

Each hotel will have its own dedicated training facility.

Dedicated training venues have also been set up at Melbourne Park, the National Tennis Centre and Albert Reserve.

Novak Djokovic, Rafael Nadal and Serena Williams are on a separate charter flight that will fly direct to Adelaide for a planned exhibition event in the South Australian capital.

AAP/ABC/AP