Novak Djokovic's visa has been cancelled and his Australian Open campaign is in tatters but two more participants may just be under the microscope. 

The tennis star was told to leave the country after he was stopped upon entering Australia despite claiming to have a vaccine exemption, sparking global controversy. 

Djokovic is reportedly in a quarantine hotel in the city for the weekend and is expected to try to challenge the decision in Victoria's courts. 

Prime Minister Scott Morrison tweeted on Thursday morning: "Mr Djokovic's visa has been cancelled. Rules are rules, especially when it comes to our borders. No one is above these rules. 

"Our strong border policies have been critical to Australia having one of the lowest death rates in the world from Covid, we are continuing to be vigilant." 

But speaking in a press conference on Thursday morning, Morrison revealed there had never been an exemption in place and Djokovic's evidence for a medical exemption was "insufficient". 

"I want to thank the Border Force officers for doing their job implementing the Government's policies ... entry with a visa requires double vaccination, or a medical exemption," he said. "I am advised that such an exemption was not in place, and as a result he is subject to the same rule as anyone else. 

"I also want to stress, that ultimately, this is the responsibility of the traveller. It is for the traveller to be able to assert and back up their ability to come into the country consistent with our laws. 

"They can take advice, but it is up to them at the end of the day, and if they do not comply with the rules, that the Australian Border Force will do their job, and they have done their job (in this case). 

"This is nothing about any one individual, it is simply a matter of following the rules, and so those processes will take their course, over the next few hours, and that event will play out as it should." 

In a potentially massive twist, Reuters and the Sydney Morning Herald have reported other participants with similar exemptions to Djokovic's have already entered the country without incident. 

Home Affairs Minister Karen Andrews has since said authorities are examining those people to ensure there have been no breaches of border rules. 

By the afternoon, the ABC's Andrew Greene tweeted: "Tennis Australia is believed to have granted exemption letters to two other individuals which don't meet Australian government entry requirements. Australian Border Force officials are now investigating their cases". 

The Sydney Morning Herald understands that of the two participants being investigated, one is a player and the other a tennis official. 

A letter detailing advice from Health Minister Greg Hunt clearly stating that people who had tested positive to Covid-19 within six months of wanting to enter the country were not deemed fully vaxxed was leaked on Thursday afternoon. It comes as Djokovic's lawyers prepared to challenge the ruling in court. 

Morrison added that it is not uncommon for travellers to try to "run the border" and that entrants to Australia need to be double vaccinated, which is assessed at the border. 

He also said it is on the traveller to have the proof to show why they have not been vaccinated. 

"He provided information to the airline to allow his entry on to the plane. But people get on the plane, that is not an assurance that they will be able to come through Australia's border at the other side," Morrison said. 

"The problem is not necessarily with the visa. There are many visas granted and if you have a visa and are double vaccinated you are very welcome to come. And I think what this says to everybody in Australia the people are welcome but if you are not double vaccinated and not an Australian resident or citizen, you cannot come." 

Morrison said Djokovic had not been singled out and he did not believe this situation would impact Australia on the global stage. 

This is despite Serbian President Aleksander Vucic vowing his nation "will fight" the "maltreatment" of the tennis star. 

"Serbia has been a good friend of Australia and provided very strong support particularly on security issues globally and we greatly appreciate that," he said. "This is a very specific case that deals with one individual and Australia's sovereign border laws and their fair application." 

Morrison said any extra scrutiny was due to Djokovic's own doing. 

"When you get people making public statements of what they say they have and what they are going to do and what their claims are, they draw significant attention to themselves. 

"Whether they are a celebrity, a politician, a tennis player, a journalist, whoever does that, they can expect to be asked questions more than others before you."