(CNN)Rafael Nadal has said that he feels "sorry" for Novak Djokovic ahead of the Australian Open but added that his rival has long been aware of the vaccine requirement to play in the tournament.
Djokovic, who is tied with Nadal and Roger Federer on 20 grand slam titles, may not be able to defend his Australian Open title after his visa to enter the country was canceled and he was blocked at the border and told he had not met the required entry rules.
The 34-year-old has not publicly revealed his vaccination status, but Australian prime minister Scott Morrison said in a news conference on Thursday that Djokovic "didn't have a valid medical exemption" to the vaccination requirement for arrivals.
Djokovic's legal team has sought an urgent injunction against the Australian Border Forces' decision to revoke his visa. The country's Federal Court has adjourned the decision until Monday on whether he will be allowed to remain in Australia or be deported, according to Reuters and public broadcaster ABC.
"He made his own decisions, and everybody is free to take their own decisions, but then there are some consequences," Nadal, who won his first singles match since August on Thursday, told reporters in Melbourne.
"Of course, I don't like the situation that is happening. In some way, I feel sorry for him. But at the same time, he knew the conditions since a lot of months ago, so he makes his own decision."
Should Djokovic, a nine-time champion at the Australian Open, not be able to compete in the tournament, then it would boost Nadal's chances of winning a record 21st grand slam title.
Daniil Medvedev and Alexander Zverev, second and third respectively in the men's rankings behind Djokovic, are also among the favorites to win in Melbourne.
"Seems some rough situation, but at the end of the day, the only thing that I can say is we have been going through very challenging [times] and a lot of families have been suffering a lot during the last two years with all the pandemic," said Nadal.
He added: "It's normal that the people here in Australia get very frustrated with the [Djokovic] case because they have been going through a lot of very hard lockdowns, and a lot of people were not able to come back home.
"From my point of view, that's the only thing that I can say is I believe in what the people who know about medicine say, and if the people say that we need to get vaccinated, we need to get the vaccine."
Nadal defeated Ricardas Berankis 6-2 7-5 to reach the quarterfinals of the Melbourne Summer Set on Thursday, a warmup tournament ahead of the Australian Open.
It was his first singles match in five months due to a left foot injury, and the Spaniard was also returning to the court having tested positive for Covid-19 in December.
"I felt so tired and I had fever," Nadal, who has been double vaccinated, said about contracting the virus.
"I had to be on the bed for a couple of days without having the chance to move much because I was very sick with fever, and one of the toughest symptoms that I had in my life in terms of flu."
Nadal is also competing in the doubles draw of the Melbourne Summer Set this week and next faces Tallon Griekspoor in the competition's singles draw on Friday.
The Australian Open begins on January 17, with the men's final taking place on January 30.
CNN's AnneClaire Stapleton and Jessie Yeung contributed to reporting.