Australian player Nick Kyrgios says his country's handling of Novak Djokovic's visa dispute has been "really bad" and urged officials to "do better".

The men's world number one, 34, is facing deportation after his entry to the country was denied on Wednesday.

"This is one of our great champions but at the end of the day, he is human," said Kyrgios.

Djokovic has launched an urgent court challenge to be heard on Monday, a week before the Australian Open begins.

Australia requires all foreign visitors entering the country to be double vaccinated. Otherwise, they need to complete 14 days in quarantine.

The 20-time Grand Slam champion, who has previously said he is opposed to vaccination, had been granted a medical exemption to play in the tournament for unspecified reasons, a decision that infuriated many Australians.

That exemption was given by two independent medical panels organised by Tennis Australia, the body that runs the Australian Open, and Victoria state, tournament organisers said.

However, the public outcry in Australia over the exemption prompted Scott Morrison, the country's prime minister, to say that Djokovic would be sent home if he failed to provide proper medical proof for his exemption.

Djokovic was quizzed for hours about his exemption evidence at Melbourne's Tullamarine Airport and is still awaiting a decision on whether he can stay in the country.

Kyrgios, 26, has been a vocal critic of Djokovic in the past.

He described Djokovic's decision to organise an exhibition event in the Balkans early in the pandemic - where the Serb and several others later tested positive for coronavirus - as "bone-headed".

When Djokovic wrote to Australian Open organisers asking them to ease quarantine restrictions for players last year, Kyrgios called him "a tool".

But the Australian has now spoken out about the treatment of the Serb as he awaits the outcome of Monday's appeal.

"Look I definitely believe in taking action - I got vaccinated because of others and for my mum's health," he wrote on Twitter.

"But how we are handling Novak's situation is bad, really bad. Do better."

American player John Isner said there is "no justification" for the treatment received by Djokovic in Melbourne.

"What Novak is going through right now is not right," said Isner, who also wrote #IStandWithNovak in the Twitter post.

"He followed the rules, was allowed to enter Australia, and now he's being detained against his own will. This is such a shame."

Canadian player Vasek Pospisil, who set up the Professional Tennis Players Association (PTPA) alongside the Serb, urged Djokovic to "stay strong".

The PTPA said it had been in close contact with Djokovic, who had "verified his wellbeing" to the organisation.

"He has also requested that we allow him to personally share the facts of his detainment in his own words, and in his own time," the body added.

Former US Open champion Marin Cilic said the episode was not good for the sport, and expressed hope that it would be resolved quickly.

"Definitely it's not a great picture for tennis," said the Croat.

Djokovic's wife Jelena thanked people for using their voice "to send love" to the player.

"We wish we were all together today, but my consolation is that at least we are healthy. And we will grow from this experience," she wrote on Instagram.

Djokovic has won the Australian Open men's singles title on a record nine occasions. The Grand Slam tournament begins at Melbourne Park on 17 January.