The visa of Novak Djokovic was revoked on his arrival in Australia.
The world number one said he was sorry for Novak, but that he could have played without a problem.
His visa was revoked when he arrived in Melbourne to defend his Australian Open title.
The exemption of the Serb to play in Australia has caused controversy.
The deportation of the man who has previously said he is against vaccinations was delayed until Monday.
Lawyers for the man lodged an urgent appeal before the delay to deportation was confirmed.
The tournament begins on 17 January.
"I think he would be playing here in Australia without a problem, if he wanted to," Nadal said.
Everybody is free to make their own decisions, but there are consequences.
I don't like the situation that is happening. I feel bad for him. He made his own decision because he knew the conditions since a long time ago.
The issue could be fixed immediately by explaining to border authorities why he is medically exempt.
He would have applied for a visa if he had a legitimate medical reason.
He said that it was not unreasonable for the authorities to check the claim.
There is a legitimate reason for that, but it doesn't need to be made public.
Nadal said that it's normal that people are frustrated.
Scott Morrison said rules are rules.
After several hours at the airport, border officials said he had failed to provide appropriate evidence for entry and would be deported.
The basis for the Serbian's medical exemption has not been disclosed by the Australian Open organizers.
The decision to allow nine-time champion Novak Djokovic to play at the tournament was met with anger in Australia, where residents have undergone some of the world's strictest restrictions during the Pandemic.
The country's prime minister, Scott Morrison, said no one was above the country's rules.
The situation is good for no-one, according to Nadal, who is level with Roger and Novak on a record 20 men's Grand Slam singles titles.
I don't have all the details, so I can't have a clear opinion on everything.
We have been through a lot in the last two years and a lot of families have been affected by the Pandemic.
The people in Australia are very frustrated with the case because they have been through a lot of hard lockdowns and have not been able to return to their homes.
Watch: Ros Atkins is on. Novak Djokovic is in Australia.
Nadal believes in medical advice on vaccinations.
He said that if the people said that they needed to get the vaccine, then it was time to get it.
That's my point of view. I went through the hospital. I've been shot twice. You don't have to worry about playing here if you do this. That is the only clear thing.
The world has suffered enough to not follow the rules. The vaccine is the only way to stop this epidemic, after a lot of people have died.
I am no-one to create a different opinion because the people who understand this say that.
The problem is not going to go away.
Russell is a tennis correspondent.
The social media post that was used to announce his medical exemption 48 hours ago was not a smart move at all and probably alerted the authorities that he was on his way.
If he turned up at immigration with minimal documents, anyone with any experience of Australian immigration will know that is very foolish and arrogant.
It will be the most remarkable story if he wins the court battle and wins the Australian Open for a 10th time. It shows how much he wants to do it.
A lot of experts thought he wouldn't have a chance of winning the case, but he is trying to stay in this immigration hotel for another four days to fight it.
This is not going to go away. He is going to have problems at other Grand Slams if vaccination is required by countries for entry. In the United States, it's hard to see how they'll give him an exemption unless he has a good reason.
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