Rafael Nadal beat Mardy Fish and secured a place in the semi-finals of the US Open for the first time in his career.

The world number one edged out the unseeded American 3-6 6-1 6-4 6-2 in a match which finished well into the early hours of Thursday morning.

Having shared the opening two sets the match was in the balance but the momentum swung Nadal's way when he broke Fish in the seventh game of the third.

He served it out and continued to hold the edge with a break in the third game of the fourth. Fish displayed his frustration at the next change of ends, smashing his racket on the floor.

The American's annoyance quickly turned to disappointment as he dropped his serve again to fall 5-2 behind.

There was no way back as the Spaniard secured the match at the second attempt when Fish hammered a forehand beyond the baseline.

"I am very happy to be in the US Open semi-finals for the first time," Nadal said. "This court has always been very hard for me."

Fish had captured the first three games in the first set en route to a 5-2 advantage.

After Nadal held, Fish was dominant on his serve and clinched the first set with a sizzling forehand winner.

"Mardy played unbelievable in the first set," Nadal said.

Next up for the 22-year-old is a last-four clash with Britain's Andy Murray, who outlasted Argentina's Juan Martin del Potro 7-6 (7/2) 7-6 (7/1) 4-6 7-5.

"He is doing very, very well," Nadal said of the Scot. "I expect a very tough match."

Nadal has been the best player in tennis this year and last month he overtook Roger Federer as the world number one, ending the Swiss star's four and a half-year reign at the top of the rankings.

The five-time grand slam champion earned that honour just a day after claiming gold at the Beijing Olympics, which marked his ATP-leading eighth title of the year.

Nadal, who also beat Federer to claim his maiden Wimbledon title in July, is now in line to become the first player since Rod Laver in 1969 to win at the French Open, the All England Club and Flushing Meadows in the same year.

He faces a tricky opponent next in Murray, however.

Facing Del Potro, who entered on a 23-match winning streak, Murray offered an array of drop shots and powerful groundstrokes against the lanky Argentinian.

Murray, who will rise from sixth in the world to fourth when the new rankings are released next week, won for the 17th time in his last 20 matches.

"I'm excited to be in the semi-final, but the tournament is still going on," he said. "When I watched the first night session of the tournament and saw all the winners of the US Open, I realised that winning is what really, really counts. I want to win.

"I don't care what happens throughout the course of a match as long as I win."

Before advancing, Murray had to overcome a third-set collapse after taking three of the first four games.

Leading 4-3, Murray had two break points but could not capitalise and he was then broken for the second time in the set as Del Potro reduced his arrears and increased his confidence.

Despite the lapse, which resulted in the Scot having to battle through a titanic fourth set, Murray was happy with his strategy.

"I knew I was going to have to fight a lot and change the pace of the ball, because he's really solid from the back of the court," he said. "My tactics worked quite well."

Murray also lost a sizeable lead in the first set and wasted a chance to serve for the second set, but still managed to keep the upper hand.

"I was in the driver's seat, up two sets," Murray said.