Defending champion Roger Federer today warned Andy Murray he faces a "very dangerous" battle against fellow Swiss star Stanislas Wawrinka on Centre Court at the US Open tonight.

Federer's difficult season came to life at the Beijing Olympics, where he partnered Wawrinka to gold medal success in the doubles tournament and he expects the second-best men's player in Switzerland to cause Murray, the sixth seed, huge problems under the Flushing Meadows floodlights.

Murray, who will break into the world's top five for the first time if he wins, will relish the challenge and has always hoped to be handed a prestigious night match on the biggest Grand Slam court in tennis. Now his wish has been granted against his friend Wawrinka, the No10 seed.

Victory would put Murray up against either 19-year-old Agentine Juan Martin del Potro or 18-year-old Kei Nishikori, of Japan, who knocked out fourth seed David Ferrer.

However, Federer said: "This is a very dangerous match for Andy because Stan is playing with a lot of confidence and it was special for both of us to win the gold medal in Beijing.

"Stan has proved himself on the big stage this year by becoming more consistent and, in the past, he used to have some dodgy matches early on in tournaments. He can now handle pressure which he didn't.

"He is unbelievably fit and if it goes the distance, then I would always back Stan to come through. I think it's also significant that the courts are a little slower here which helps him because he has really improved on hard courts."

Murray has played Wawrinka four times this year - they are tied 3-3 during their professional careers - and their friendship will be put to one side tonight.

Murray, who beat Jurgen Melzer 6-7, 4-6, 7-6, 6-1, 6-3 in the third round, said: "I have always wanted to play a night match on Centre Court here. I first saw the court when I sneaked in after winning the Boy's title in 2004 and watched some of the women's final.

"For Stan, this has been his best year by far. He started last year pretty well and then had a bad injury to his knee and had to have surgery. He has played great this year.

"My matches against him have all been close, none of them has been comfortable. For me, there is a chance that I could win the tournament. So I'm going to give it my best shot.

"That's my goal for the tournament, to try to win it. I think it's better to set the bar high and maybe you don't reach it.

"I might not necessarily win but if I go with the intention of doing that, it might not come as such a surprise if I do go deep. There are a lot of different emotions going through your head when you win a five-setter like that against Melzer."

Wawrinka is up for the battle and said: "Sometimes you can win a game against Andy easily but then he comes right back at you.

"But the one thing I like about him is that you can go on court, shout 'Come on' in his face and after the match, you can talk to each other."

For Federer, the fourth round will bring a clash with Russia's Igor Andreev, the 23rd seed who knocked out Fernando Verdasco yesterday. Federer was on court for barely an hour as he destroyed Radek Stapenek 6-3, 6-3, 6-2 and is refusing to give a verdict on a season that has yet to bring a Grand Slam title.

He said: "See what happens here in a week's time and we'll see how the indoor season goes. I won an Olympic gold and have played solid throughout the year, so it's not the time to analyse a season yet."

Eighth seed Andy Roddick defeated Andreas Seppi 6-2, 7-5, 7-6 while Novak Djokovic, the third seed, overcame the dangerous Marin Cilic 6-7, 7-5, 6-4, 7-6.

Britain's Daniel Cox fought back to register an important US Open Boys first-round win over eighth seed Henrique Cunha, of Brazil 5-7, 7-6, 6-4.