World number one Rafa Nadal came from behind to dispatch underdog Taro Daniel of Japan 4-6 6-3 6-2 6-2 in front of a sold-out crowd at the U.S. Open on Thursday.

Despite playing a marathon five-setter the day before, 121st ranked Daniel looked the fresher of the pair at the outset of the match, breaking Nadal to take a 5-4 advantage when the Spaniard dumped a forehand into the net.

He eventually won the set and appeared poised to deliver a massive upset.

Rafael Nadal of Spain (right) shakes hands with Taro Daniel of Japan after their match.

But Nadal soon found the accuracy that had eluded him in the first set and his booming serve proved too much for the exhausted 24-year-old, who is now 0-6 against top 20 opponents in his career.

"All the matches are difficult, especially here where everyone wants to play their best," Nadal said in an on-court interview at the Arthur Ashe Stadium after the win.

"It's true that I didn't play very well tonight but it's also true that I'm through to the next round."

Nadal is now one step closer to a potential semi-finals match against third seed Roger Federer, who won his second consecutive five-set match earlier on Thursday.

"I am here to improve every day. I believe I can do it much better and I really feel I am going to do it much better," Nadal said.

Next up for the 31-year-old is a third round match against aggressive Argentinian Leonardo Mayer.