The Spanish second seed, a three-time winner at Flushing Meadows, will face Russian Daniil Medvedev on Sunday after a straight-sets win over Berrettini.
On a wet and windy night in New York, a storm briefly threatened to brew under the roof as Berrettini, in his first major semi-final, had two points for the opening set.
The Italian 24th seed had been clinging on to Nadal, fending off six break points including a set point.
But the underdog then raced into a 4-0 lead in the tie-break, and a drop shot followed by a volley right on to the baseline gave him his big chance.
However when another volley hit the tape, and one drop shot too many fell short, that chance evaporated.
Every service game was a trial for Berrettini, who saved more break points in the first and third games of the second set but succumbed to trail 5-3.
Two sets to the good, Nadal disappeared into the distance and a 7-6 (6) 6-4 6-1 victory was wrapped up in two hours and 35 minutes.
The 33-year-old will move to within one major title of Roger Federer's record tally of 20 if he can overcome Medvedev.
Nadal said: "First set was a little bit frustrating, I had a lot of break points and you don't want to be in a tie-break with a player like Matteo.
"In the tie-break I was a bit lucky but I survived. Finally I had a break in the second and the match completely changed. I'm super happy to be back in the final of the US Open.
"Congrats to Matteo. He's young, he has everything and he has a great future in front of him.
"Daniil is one of the most solid players on the tour, he's making steps every week and he had an amazing summer. He will be the toughest opponent and I will need to play my best."
Earlier, Medvedev secured a place in his first grand slam final and won over a hostile crowd in the process
The hot-headed Russian, who has defiantly blasted his way through the draw to a backdrop of jeers from the stands, overcame Grigor Dimitrov.
Medvedev became the villain of the piece in the third round when he snatched a towel from a ballboy and then gave the crowd the middle finger, before goading them in his post-match interviews throughout the fortnight.
But after beating Dimitrov in straight sets to make Sunday's showpiece, the 23-year-old declared: "I love the USA!"
Dimitrov, who knocked out Roger Federer, will wonder how he lost the opening set, while the second set featured some of the most compelling tennis of the championships.
Medvedev took that one too and went on win 7-6 (5) 6-4 6-3 victory in two hours and 38 minutes.
"The tournament of controversies," he smiled afterwards. "Finally I'm here after three sets. I'm just happy to be in the final.
"When I was going to the USA, I didn't know it would be this good. I have to say I love the USA!
"I want to be a better person than I was a few days here."