Veteran American becomes second top 30 player to skip Australian Open

Former world No.8 Isner, who has a two-year-old daughter and one-year-old son, said he had chose to stay home as health guidelines and restrictions on entourages meant he would have to be away from his family for an extended period.

Melbourne was the epicentre of Australia's largest second wave outbreak of coronavirus, which prompted strict lockdown measures for four months.

John Isner will not play the 2021 Australian Open.

There is also ongoing debate about the Australian Open tennis tournament, with the first of the global stars competing set to touch down in Melbourne early Thursday evening on a charter flight.

Players will be allowed five hours outside their rooms each day for practice and exercise at dedicated training bubbles and will be tested daily. But though the rules are definitely harsh, said defending Australian Open champion Sofia Kenin in response to the latest restrictions, it's for the protection of and in the best interests of everyone involved.

There is also the threat of sanctions, including hefty fines, spending additional time in quarantine or deportation, if the rules are broken.

"The problem is that Mirka (Federer's wife) and their children couldn't leave the room", Tennis Australia's head of player liaisons Andre Sa told Brazilian media.

The 35-year-old Isner acknowledged that the decision to withdraw from the 2021 Australian Open was a tough one.

Tennis Australia initially wanted players arriving in Melbourne by mid-December. "The exception is only for players".

However, he then said he was able to board the flight on Wednesday, which is carrying other players and coaching staff to the first Grand Slam of the year, with the help of Australian Open tournament director Craig Tiley.

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.

"It's just insane, we've never seen anything like it", he told the Tennis Channel.

Private workplaces can return to 50 per cent capacity from Monday, while public service offices can return to 25 per cent capacity. "Hopefully there will be normalcy to it".