Nick Kyrgios surprisingly came to the defence of Novak Djokovic Friday, calling Australia's treatment of the world number one "really bad" and urging authorities to "do better".
The controversial Australian has previously been one of the Serbian's staunchest critics, at one point labelling him a "tool" over his response to the coronavirus pandemic.
But with Djokovic -- who has declared himself against vaccines and claimed to have an exemption to Australia's pandemic entry restrictions -- in an immigration detention hotel, Kyrgios blasted the way he had been dealt with.
"Look I definitely believe in taking action, I got vaccinated because of others and for my mum's health," he said on social media. "But how we are handling Novak's situation is bad, really bad.
"Like these memes, headlines, this is one of our great champions but at the end of the day, he is human. Do better."
Djokovic was detained on arrival in Australia earlier this week with his visa revoked for failing to meet the tough Covid restrictions.
Those granted permission to enter the country must prove they are fully vaccinated or have a doctor's medical exemption, with authorities saying he provided evidence of neither.
He won a legal reprieve from deportation until at least Monday, when his case will be heard in court.
Kyrgios has waged a running battle with the Serbian in recent years, sparked by Djokovic's ill-fated Adria Tour exhibition in 2020 as the coronavirus pandemic raged.
He lashed his "stupidity" when social distancing guidelines were flouted, which led to multiple cases of coronavirus.
And last year, the Australian dismissed him as a "tool" after he issued a list of demands for quarantined players who arrived on charter flights for the Australian Open.
Djokovic responded by saying he has little respect for Kyrgios's off-court behaviour.
Meanwhile, former US Open champion Marin Cilic said it was "incredible" that Djokovic was in immigration detention and it wasn't a good look for international tennis.
"Looking at the situation, it's definitely incredible that this happened the way it did, especially to Novak, that he got here, that this is still going on," he said at the Adelaide International.
"Definitely feeling very sorry for him. Hope that this is going to be resolved very soon. Definitely it's not a great picture for tennis, to have something like this."