Murray in cruise control

The Scot, seeded sixth, was in dominant form as he cruised past Argentine Sergio Roitman 6-3 6-4 6-0 to set up a match with Frenchman Michael Llodra.

At the end of last year, Murray went to Florida in the off-season and increased the intensity of his training, working hard in the gym to lift his fitness levels.

The work has already paid off with three titles in 2008, including his first Masters Series crown in Cincinnati last month.

His victory over world number three Novak Djokovic, in the Cincinnati final, was his second over the Serb in successive weeks, having lost the first four matches against him.

"I think a lot of things go into having the ability to win a slam," Murray said.

"Talent gets you to a certain level and then the hard work starts to kick in, and if you're not putting in the hard work, you're not going to get the opportunities to win a grand slam.

"I've really stepped up my work load off the court and started travelling with a fitness trainer. I think that's the big difference in my game and the reason why I have the potential to come through and win a slam."

Roger Federer's relative slump this year - the former world number one has not won a grand-slam title for the first time since 2002 - has left the men's event far more open than in recent years.

Nadal, Djokovic and Murray are the leading challengers to take the U.S. Open crown from Federer and the Scot said more players now believed the Swiss could be beaten.

"I think even though Federer was still the favourite for Wimbledon, there were still guys that had a chance of doing some damage there," Murray said.

"I think it's really exciting for tennis. There's obviously quite a few guys that have the potential to win it.

"I'd still say Federer is one of the favourites but, in terms of being unbeatable, I think that's a mindset that isn't great if you're going on the court believing that you can't win against someone.

"I think it's taken a few of the top guys to see Federer lose to lower-ranked players to start believing they can win against him."

"I still think Federer is playing great tennis, but the mindset of the other players has changed because he's lost more this year than he has in the past."