The 35-year-old Spaniard fought back from two sets down to beat Russia’s Daniil Medvedev in Melbourne.
Last year, Nadal feared his career was over because of a foot injury.
“If you put everything together, it has probably been the biggest comeback of my tennis career,” he said.
Chronic pain in Nadal’s left foot restricted him to only one tournament in the final seven months of 2021, while a bout of coronavirus in mid-December also left him “very sick with fever”.
Those setbacks meant the Australian Open was just his second competitive event in five months, having won a warm-up tournament at Melbourne Park earlier in January.
“For the last six months, I really fought a lot to try to be back on court,” Nadal said.
“There have been very, very tough moments. There have been conversations, tough ones, because you don’t know if I was going to have the chance to be back on the tour.”
In the absence of the deported Novak Djokovic and the injured Roger Federer in Melbourne, Nadal has moved one ahead of his great rivals in the all-time list of players with the most major men’s singles titles.
‘I don’t care much’ about record
While eager to win as many Grand Slam titles as possible, Nadal has often said he is not motivated by beating Djokovic and Federer’s totals.
After the Australian Open final, he said: “Of course, for me it’s amazing to achieve another Grand Slam at this moment of my career. Of course, I know it’s a special number, 21.
“I know the big significance of this title. I feel lucky to achieve one more very special thing in my tennis career.
“But I don’t care much if I am the best in history, one of the best in history, or not the best in history. Honestly, I don’t care much.”
Djokovic congratulated Nadal on his “amazing achievement” and “impressive fighting spirit”, while Federer offered his “heartfelt congratulations on becoming the first man to win 21 Grand Slam singles titles”.
The Swiss player added: “A few months ago we were joking about both being on crutches, amazing. Never underestimate a great champion.”
Sunday’s win was Nadal’s second Australian Open title, and came 13 years after his first.
His 2-6 6-7 (5-7) 6-4 6-4 7-5 victory against US Open champion Medvedev lasted five hours and 24 minutes. When the second seed could not return a net volley on the first of the Spaniard’s three match points, it was 01:11 local time on Monday in Melbourne.
It was the second longest Grand Slam final in history, 30 minutes short of the 2012 final at Melbourne Park, when Nadal lost to Djokovic.
Nadal had to sit down on a chair when this year’s trophy presentation began, echoing the scenes in 2012.
“It is the most unexpected title, without a doubt. And most surprising, I think, for everyone,” he said.
“It has been a very emotional night. Even now I am destroyed, honestly, physically. I can’t think much. I can’t remember a lot of moments of the match.”