Djokovic toils as Federer cruises

World number three Novak Djokovic was tested to the limit before scraping into round four of the US Open after a four-hour battle with Marin Cilic.

Cilic, 19, won the first set and saved two match points in the fourth before losing 6-7 (7-9) 7-5 6-4 7-6 (7-0).

Former world number one Roger Federer cruised through, barely breaking sweat as he beat Radek Stepanek 6-3 6-3 6-2.

Home favourite Andy Roddick also showed something like his best form as he saw off Andreas Seppi 6-2 7-5 7-6 (7-4).

Cilic won a high-quality first set on a tie-break, bravely smacking a clean backhand winner down the line to snatch it.

Both men then fluctuated between the sublime and the distinctly average, with Djokovic allowing an early break to slip in each of the next three sets.

| It's good for me to not waste any energy Roger Federer

An inspired Cilic survived two match points before levelling at 5-5 in the fourth but he crumbled in the tie-break and Djokovic prevailed after three hours 52 minutes.

The Serbian admitted: "I was really trying to stay in the match all the time.

"It is always good in the early stages to have long matches, to hustle and stay on the court a long time. Now I'm motivated to go far.

"I always manage to get through these tough matches. If you are mentally able to play the best tennis in the most important moments you are different than the others.

"The good thing about that second set was that I stayed focused. He was stepping in and I was playing a bit too defensive.

"He didn't have much to lose as a youngster. He won at New Haven (one week before the US Open) and he won a couple of matches here and he was on the centre court for the first time.

"It was more the fight and focus I had in the end.

"I got myself together after those two match points. I was mentally strong. For me this match is extremely important mentally for me to continue going on."

Djokovic will now meet Spain's Tommy Robredo in round four.

"He is another tough opponent. If I'm fresh enough mentally and physically I think I can get a positive outcome," Djokovic said.

"I can expect a big fight. He's confident. The good thing about Grand Slams is you have a day off. You can recover. I'm sure I'll be recovered enough."

Robredo earlier beat Australian Open finalist Jo-Wilfried Tsonga 7-6 (7-2) 6-2 6-3.

Roddick, who was the last man to win the US Open before four-times champion Federer, will next face Chilean 11th seed Fernando Gonzalez , a 7-5 6-4 6-7 (3-7) 6-1 winner over Finland's Jarkko Nieminen.

Fifth seed Nikolay Davydenko , a possible quarter-final opponent for Federer, brushed aside fellow Russian Dmitry Tursunov 6-2 7-6 6-3 to set up a fourth-round match with Gilles Muller .

Muller, the qualifier from Luxembourg, continued his incredible run with a 6-7 (3-7) 3-6 7-6 (7-5) 7-6 (8-6) 7-5 win over 18th seed Nicolas Almagro.

The 25-year-old, who won the junior event at Flushing Meadows in 2001, had not won three successive Tour level matches since July 2005.

The qualifier also came from two sets down to beat Tommy Haas in the previous round.

Federer will face Igor Andreev next after the Russian defeated Spain's Fernando Verdasco 6-2 6-4 6-4.

Defending champion Federer is yet to drop a set in his first three matches and victory over Stepanek was his 30th straight win at Flushing Meadows.

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The 27-year-old is seeking to become the first player since Bill Tilden in 1924 to win five US titles in a row.

His last defeat came in 2003 when he lost to David Nalbandian in the fourth round.

He showed no sign of vulnerability on Sunday, with his serve firing particularly well as he avenged his defeat by Stepanek in Rome earlier this year.

"It's good for me to not waste any energy," said Federer.

"I'm playing well and moving on in the draw.

"I think all in all I'm really happy. I've been serving well for the first three rounds, and that's always a good sign for the rest of the tournament.

"I don't try to impress anybody in the early rounds. If it happens, that's great. I don't really care that much."

Roddick, the eighth seed, believes he too is hitting top form after some mixed results in the lead-up to his home Grand Slam.

The 26-year-old, who landed 73% of his first serves, was broken for the first time at 2-2 in the third set before hitting back to take it on a tie-break.

"For the first time in a little while, I feel like I'm match tough," he said.

"I've gotten in some tough moments here and played my way out of them pretty good, so that's a good sign.

"You're always moving in one of two directions and I'm going in the right one right now."