By Pritha Sarkar

NEW YORK (Reuters) - Novak Djokovic better have his heavy amour on when he steps out to stop four-times champion Roger Federer from reaching his fifth successive final at the U.S. Open on Saturday.

The Serbian is likely to be greeted by 23,000 hostile fans on Arthur Ashe Stadium after his astonishing outburst against American Andy Roddick following their fourth-round tussle.

Irked by Roddick's suggestion that he was a hypochondriac, Djokovic told the booing crowd on Thursday: "Andy was saying that I have 16 injuries in the last match so obviously I don't, right? That's not nice anyhow to say in front of this crowd that I have 16 injuries and I am faking it."

Less than an hour later, and realizing that he had overreacted, Djokovic tried to do some damage limitation.

"He made a joke and it was a misunderstanding, so I don't blame it on him. If I exaggerated on the court today and I made maybe a mistake (by) saying that in front of 20,000 people, I apologize. This was just impulsive.

"So I just hope people understand."

Whether the fans will accept his apology remains to be seen but with Federer being a firm crowd favorite, the Serb is unlikely to be Mr. Popular on Saturday.

In the other semi, Rafael Nadal, who deposed Federer as world number one last month, will take on Briton Andy Murray.

In a year when Federer's aura of invincibility has been pricked, punctured and ripped apart, the U.S. Open remains his final chance to rescue what has been a dreadful season by his own high standards.

The winner of 12 grand slam titles at the start of 2008, he relinquished his Australian Open crown to Djokovic. He then received a mauling by Nadal in the French Open final but the most painful defeat was undoubtedly at the All England Club.

Going for a record-equaling sixth successive men's title, he ended up playing the supporting act to his bogeyman Nadal in what is considered by many as the greatest tennis match ever.

While New York fans are desperate to see another final showdown between the Swiss master and Majorcan muscleman, Djokovic hopes he can turn out to be the joker in the pack.

TROPICAL STORM

The 21-year-old snapped Federer's record streak of reaching 10 successive major finals in January at Melbourne Park, and despite his trials and tribulations on court and his losing 6-2 record against the Swiss, he feels he could repeat the feat.

"That (win in Melbourne) was one of my best matches. Hopefully I can do the same. I just need to be myself and need to be aggressive," said Djokovic, the 2007 runner-up here.

With a tropical storm forecast to wash out Saturday's program, Djokovic also felt that could help his cause in recovering from his exhausting run. In the past six days he has been engaged on court for almost 10 hours.

"Physically I've been struggling... so any extra day of relaxation and just recovery would be good for me," he said.

Federer was banking to capitalize on his Open experience.

"He's been very consistent with his young age... (but) he's never won the U.S. Open, so I have that over him," he said.

Nadal and Murray are in unfamiliar territory since both have reached the last four in New York for the first time.

In Murray's case, it is his first grand slam semi-final and if he wants to become the first Briton to reach the final since Greg Rusedski in 1997, he will have to overcome a 5-0 losing record against the irrepressible Spaniard.

"The one thing that I need to improve is to return better," acknowledged the 21-year-old Scot.

"The first couple of times I played him I returned very well. The last two or three matches I didn't return well at all. That's the only thing that I need to do."

Murray will probably discover that what looks simple on paper may be more difficult when he faces Nadal across the net.

In a season when Nadal finally spread his tentacles outside Roland Garros, by grabbing his first Wimbledon title and an Olympic gold, the U.S. Open title is now his next target.

Seeded number one at a slam for the first time, he would like nothing better than fulfilling his billing and Murray will have to play the match of his life if he is to hand the Spaniard only his third defeat in his last 57 matches.

"My goal is find my best feeling and my best performance (then I will have) all the chance for winning," he said.

(Editing by Larry Fine)