Nadal already has Paris Open and Wimbledon titles and a semifinal slot at the Australian Open under this belt this year -- results that allowed him to overtake Roger Federer for the top spot in men's tennis.

However, Fish, unseeded but on a good roll of his own, came out looking for the upset and tested Nadal before the Spaniard claimed a 3-6, 6-1, 6-4, 6-2 victory that ended early Thursday.

"For me it was a very important win," said Nadal. "Very happy to be in the semifinals, for sure. That's another important thing in my career -- play the semifinals in all Grand Slams in the same season."

Nadal next faces sixth-seeded Andy Murray, who reached the semifinals with a tough four-set victory over 17th-seeded Juan Martin del Potro. Murray secured his first spot in a Grand Slam semifinal with a 7-6 (7-2), 7-6 (7-1), 4-6, 7-5 victory in a match that took nearly four hours to complete.

Del Porto saved one match point before firing a shot wide, ending a 23-match winning streak that included titles in his last four tournaments. Murray recorded nine aces and survived 61 unforced errors.

The other two quarterfinals will be played Thursday. Federer, the four-time defending U.S. Open champion, will meet Gilles Muller and third-seeded Novak Djokovic will face Andy Roddick.