Top-seeded Nadal, who was pushed to two tiebreakers by German qualifier Bjorn Phau in the first round, blasted his way past DeHeart, losing serve just once in the battle of left-handers that closed the fourth day of play.
The 22-year-old Spaniard was in total command, landing 70 percent of his first serves and facing just one break point from the DeHeart, who cashed it in and led 3-0 in the third set.
Nadal brought the 24-year-old American, ranked 261 and playing in his first grand slam, back to earth by winning the next five games and then closing out the match with a sizzling forehand winner to serve out his victory.
"The important thing was I had to come back in the third set and I'm happy with the victory," said Nadal.
The Spaniard had complained of fatigue after his opening match on Monday, a hangover from a busy season that included title runs at the French, Wimbledon and Beijing Olympics.
He was full of energy against DeHeart on the Arthur Ashe Stadium court, needing only 19 minutes to claim the first set and less than an hour to take a two-sets lead.
"I forgot a little bit about tennis," the revitalised Spaniard said about his time off. "I played golf on Tuesday.
"Now I'm in the third round. I'm happy. For me it's a very important tournament here," said Nadal, who was eliminated in the fourth round last year.
Nadal, winner of 40 of his last 41 matches, next meets 71st-ranked Viktor Troicki of Serbia.
"I have to improve for the next round," he said. "I have a tough opponent, Troicki. He is a good server."
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