Djokovic, who has won a record nine Australian Open titles, was detained at the Melbourne airport on Wednesday evening and his entry visa, supported by an medical exemption from the country's vaccination requirements, was revoked.
Federal Border Force officials at the airport said Djokovic was unable to justify the grounds for his exemption.
Chasing a men's record 21st Grand Slam, Djokovic is holed up in immigration detention as his lawyers battle to secure his release to play in the Australian Open and be exempt from strict Covid-19 vaccination requirements.
Kyrgios, who labelled Djokovic a "tool" after the Serb sought to improve quarantine conditions for players ahead of last year's Grand Slam, said the situation had been handled poorly.
"Look I definitely believe in taking action, I got vaccinated because of others and for my mum's health, but how we are handling Novak's situation is bad, really bad," tweeted Kyrgios.
"Like these memes, headlines, this is one of our great champions but at the end of the day, he is human. Do better."
"What Novak is going through right now is not right," Isner said on Twitter. "There's no justification for the treatment he's receiving ... This is such a shame."
Former US Open champion Marin Cilic hoped the situation would be quickly resolved and said that the whole episode was not good for the sport.
"Definitely, it's not a great picture for tennis," Cilic told reporters in Adelaide on Friday.
The Professional Tennis Players Association (PTPA), the breakaway players' body launched by Djokovic in 2020, said in a statement that they were in close contact with him.
"Djokovic has verified his well-being to us," the PTPA said in a statement. "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."
Djokovic has consistently refused to disclose his vaccination status while publicly criticising mandatory vaccines.