Rafael Nadal says he feels sorry for Novak Djokovic after he was denied entry to Australia, but added the world number one could be playing "without a problem" if he had wanted to.
Djokovic was due to defend his Australian Open title but had his visa revoked on arrival in Melbourne.
The Serb's vaccine exemption to play in Australia has caused controversy.
Djokovic, who has previously said he is opposed to vaccination, has had his deportation delayed until Monday.
The 34-year-old is being held in a government detention hotel, his lawyers having lodged an urgent appeal before the delay to deportation was confirmed.
The tournament starts on 17 January.
"I think if he wanted, he would be playing here in Australia without a problem," Nadal said, following victory at a warm-up tournament in Melbourne.
"He made his own decisions, and everybody is free to take their own decisions, but then there are some consequences.
"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."
Djokovic could "fix" the issue immediately by explaining to border authorities why he is medically exempt, according to MP for Australia's governing Liberal Party Warren Entsch.
Entsch told BBC Radio 5 Live that Djokovic would have applied for his visa on the basis he "had a legitimate medical reason".
"It's not unreasonable for the authorities to check the veracity of that claim," he explained.
"It doesn't need to be made public, but they've got to see that the visa that was issued - there is a legitimate reason for that."
'It's normal that people are frustrated' - Nadal
PM Scott Morrison on Djokovic: "Rules are rules"Djokovic was held at Melbourne airport for several hours before border officials announced he had "failed to provide appropriate evidence" for entry and would be deported.
Australian Open organisers said the Serb's medical exemption had been granted by two independent medical panels organised by Tennis Australia and Victoria state, but the basis for the exemption has not been disclosed.
The decision to allow nine-time champion Djokovic to play at the tournament was met with anger in Australia, where residents have endured some of the world's strictest restrictions during the pandemic with some still barred from travelling between states or globally.
The country's prime minister, Scott Morrison, denied Djokovic was being singled out and said no-one was above the country's rules.
Nadal, level with Djokovic and Roger Federer on a record 20 men's Grand Slam singles titles, described the situation as "good for no-one".
"I can't have a clear opinion on everything because I don't have all the details, honestly," he continued.
"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.
"It's normal that the people here in Australia get very frustrated with the 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."
Watch: Ros Atkins On... Novak Djokovic and AustraliaNadal, who contracted coronavirus in December, added he "believes in" medical advice on vaccination.
"If the people say that we need to get vaccinated, we need to get the vaccine," he said.
"That's my point of view. I went through the Covid. I have been vaccinated twice. If you do this, you don't have any problem to play here. That's the only clear thing.
"The world in my opinion has been suffering enough to not follow the rules. After a lot of people have been dying for two years, my feeling is the vaccine is the only way to stop this pandemic.
"That's what the people who understand this say, and I am no-one to create a different opinion."
US Open champion Daniil Medvedev, who will be the Australian Open number one seed if Djokovic does not play, says it is "tough" to know what should be done.
"If he had a fair exemption from the rule, well, he should be here; if he didn't, he shouldn't be here," Medvedev said.
"It sounds easy, but it seems very tough in real life, and I don't know the insides of the story, so that's why I'm just focusing on myself."
'This is not a problem that is going to go away' - analysis
BBC tennis correspondent Russell Fuller
Djokovic is being used as a political football, but the social media post he sent to announce his medical exemption 48 hours ago was not a smart move, and probably alerted the authorities he was on his way.
If, as we are told, he turned up at immigration with minimal documentation then - as anybody with any experience of Australian immigration will tell you - that is very unwise, and actually quite arrogant.
If he wins the court battle, he stays, and if he wins the Australian Open for a 10th time, it will be the most remarkable story. It shows how much he wants to do it.
He is putting himself through four more nights in an immigration hotel to fight a legal case in which the Australian government is his opponent.
This is not a problem that is going to go away. As long as countries demand vaccination as a condition of entry, Djokovic is going to have problems at other Grand Slams.
It is hard to think the United States will offer him a medical exemption without an exceptionally good reason when the US Open rolls around at the end of August.
