Roger Federer was forced to wait to learn his US Open final opponent when rain took Andy Murray and Rafa Nadal off court.

The Swiss continued his bid for a fifth straight title at Flushing Meadows with a 6-3 5-7 7-5 6-2 victory over Novak Djokovic in the first of Saturday's semis.

Organisers tinkered with the schedule to avoid the inclement weather from Tropical Storm Hanna, but there was not time to squeeze both matches in before the heavens opened with Murray on the brink of shocking the world number one at 6-2 7-6 (7/5) 2-3.

Play was suspended for the remainder of the day, with the Nadal-Murray match to be completed on Sunday and the men's final moved to Monday evening.

Playing in his first Grand Slam where he was not ranked number one in the world since the Australian Open in 2004, Federer started quickly and kept up the pressure on Djokovic.

Federer finished with 13 winners against just three unforced errors in wrapping up the opening set in 25 minutes.

"It was important to stay grounded because I knew that tough times were going to arrive, and that's exactly happened in the second set," Federer said of his success early on.

The second seed finished up with five breaks en route to capturing his 33rd consecutive win at Flushing Meadows.

The Swiss star also had 20 aces compared to six for Djokovic and edged the Serbian in winners with 51 to 43.

Despite his roll here, Federer has yet to claim a hardcourt title this season, a slump that has allowed Nadal to overtake him for the world's top ranking last month, snapping the Swiss star's record run of 237 straight weeks.

Surprisingly, Federer had a chance to drop to third in the world if Djokovic had won their matchup and then gone on to claim his second Grand Slam title of the year. The Serbian beat Federer in the semis en route to the Australian Open crown in January.

Another win for the Serb seemed out of the question from the start. Even though Djokovic did even the match at a set apiece, Federer took control for good following a break in the 11th game of the third before going on to serve out the set.

In the fourth, the Swiss star looked close to his dominant best.

"I had moments out there where I really felt, this is how I normally play on hardcourt," Federer said. "Half volleys, passing shots, good serving, putting the pressure on, playing with the wind, using it to my advantage. All those things.

"I definitely had moments where I thought, this is how I would like to play every time. So it was a very nice feeling, actually, to get that feeling back."

The runner-up here last year, Djokovic dropped has now lost seven of nine career matches against Federer.

While the former number one looked somewhere near his best, the world's current best player was not allowed to find his best form by an on-form Murray.

The Scot had lost all five of his previous meetings with Nadal, and had not won a set against the Spaniard since their first encounter at the 2007 Australian Open, most recently losing in the Wimbledon quarter-finals and in Toronto.

Yet the sixth seed had two in the bag before the rain came at the Billie Jean King National Tennis Centre.

The match had been scheduled to take place on Arthur Ashe Stadium following the first semi-final but the impending weather meant Murray and Nadal were switched to the second show court on Louis Armstrong Stadium as play began while Federer and Djokovic were still playing next door.

Murray, a three-time ATP Tour winner this year, won his first service game to love and earned his first break in the next game.

Nadal, the reigning French Open, Wimbledon and Olympic champion, had claimed he had not yet been at his best in New York as he continued to recover from his recent success in Beijing.

Murray, though, was in confident mood and he claimed a second break to take a 5-2 lead and serve for the set, which he claimed with a winning volley.

Nadal upped his game in the second set, holding off three break points in the third game.

Murray continued his attack of the left-hander's serve, forcing the Spaniard to save two more break points in the fifth game.

Murray's first service was at the best it had been all tournament, the end of the eighth game seeing him reach a 73% first service for the second set, 63% for the match.

As if to underline the point, he won his next service game to love with an ace down the middle to level the set at 5-5 on serve.

The set went into a tie-break, with Murray twice failing to capitalise on mini-breaks, but he had some luck when Nadal saw a return of his second serve bounce off the top of the net and fall the wrong side and he gained set point at 6-5 with Nadal serving and sending a backhand wide.

Murray clenched his fists in delight as he took a firm grip on the match at 2-0 up.

Nadal showed he was not giving up on his quest for a third grand slam of the year when he took the first game of the third set by breaking Murray's serve but the weather prevented him making any further inroads into Murray's lead.