Roger Federer made a slow start in New York, losing the first set at the Rod Laver Arena.
Federer made a fast start, winning his first service game, before collapsing throughout the rest of the set.
The Swiss icon went down 6-4 after the opening set, as 38-year-old Nagal appeared to be on his way to a stunning upset win.
And speaking to ESPN afterwards, he admitted that he felt he had played poorly during the opener.
“I just tried to forget it, play tough, and stay with him,” Federer said.
”It was a tough first set for him, because I had the lead, things were looking good, but he came back and played strong.
“Credit to him for playing a solid first set, I was missing a lot of balls and trying to cut down on the unforced errors, and hopefully serve a bit better, because that let me down.
“Luckily it all came together in the end, and I played some good tennis, so I’m happy.”
But he battled back with a superb second set, winning 6-1 in sublime style, before taking the third 6-2.
In the fourth, Federer continued to raise his level, despite suffering a late scare in the final set.
He admitted afterwards that he had to reassure himself after surviving from four break points while serving out the match to win 4-6, 6-1, 6-2, 6-4.
Federer said he had to tell himself: “Here come the serves, hopefully, and a few forehands if possible.
“I’ve been in that situation many, many times before, look, if I get broken I’m still at 5-5, it’s two sets to one, it’s all good, it’s more of a mindset.
“It was nice to serve it out there.
“I thought I played like my beard today, a little bit rusty.
“I’m going to shave it off for the next match, it’s better.”