Having scraped through a five-set contest with Denis Shapovalov in the quarter-finals, there were doubts over how well Nadal would have recovered.
Playing under the roof on Rod Laver Arena while thunder rolled around Melbourne the Spaniard was in complete control for two sets and recovered from losing the third to claim a 6-3 6-2 3-6 6-3 victory.
Nadal, who feared a foot problem last year would end his career, is now only one match away from becoming the first man to win a 21st grand slam singles title.
The 35-year-old, whose only Australian Open title came back in 2009, said: “I started the match playing great, first two sets have been one of the best since a long time.
“I know how good is Matteo. In the third I knew at some point he’s going to go for the shots. I think I didn’t play a good game with my serve but he played some great shots.
“Then we need to suffer and we need to fight. That’s the only way to be where I am today. It means a lot to me to be in the final again here.”
Seventh seed Berrettini began nervously in his third grand slam semi-final and found his weaker backhand ruthlessly exploited by Nadal, who seemed to be cruising into the final.
Berrettini, the beaten finalist at Wimbledon last year, did not manage to apply any pressure or force his opponent to leave his comfort zone for two sets but in the third things slowly began to turn.
The Italian had played Nadal once before, at the US Open in 2019, and had not created a break point. Finally he managed it in the eighth game of their sixth set.
Berrettini has one of the biggest forehands in the game and a memorable running pass helped him to 0-40 before he clinched the break with another winner.
Nadal had been dominant against Shapovalov at two sets to love up before things became extremely complicated, and the sixth seed was looking significantly less sharp physically early in the fourth.
He dug deep to stay on terms and, when he sensed an opening on the Berrettini serve, he pounced, the Italian saving one break point at 3-4 after the rally of the match only to dump a forehand in the net on the second.
Berrettini has not yet beaten a top-10 opponent at a slam, and the 25-year-old clearly has areas of his game to work on, but this was Nadal’s day.
On the chance of making history with his 21st slam title, Nadal said: “For me it’s all about the Australian Open more than anything else. It’s an amazing event. I’ve been unlucky with a few injuries. I was close a couple of times. I never thought about another chance in 2022.”
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