Nadal has never been beyond the third round in New York

The 20-year-old Spaniard dropped the second set against world number 61 Horna but eventually came through for a 6-4 4-6 6-4 6-2 victory.

"He was beginning to hit the ball harder in the second set and I had some problems so I had to play more aggressive," said Nadal.

Lleyton Hewitt, seeded 15th, had a 6-4 6-2 6-2 win over Jan Hernych.

The Australian admitted he was relieved not to have too hard a match as he is still struggling with a knee injury.

"I felt like when the draw came out that I was going to have some pain in that first match and I had to get through it," said the 2001 champion.

"It was more an amount of how I was going to have to recover after that. It didn't get any worse. Obviously I'm happy with that.

"I have a day off on Friday to do all the right things and give myself every possible chance of playing well on Saturday."

Sixth seed Tommy Robredo was the first man to reach the third round, beating Korean Lee Hyung-taik 3-6 6-3 6-4 6-1.

Robredo now plays Stanislas Wawrinka, who staged an amazing comeback to beat Swede Robin Soderling in five sets.

The Swiss player saved a match point at two sets and 4-5 down before taking the third set on a tie-break.

He then raced through the fourth and fifth sets, dropping just one game to score a 6-7 (4/7) 2-6 7-6 (7/4) 6-0 6-1 victory.