Four-time defending champion Roger Federer and 2007 runner-up Novak Djokovic survived five-set scares from stubborn fourth-round opponents on Tuesday at the $20.6 million US Open.

Federer, whose 12 Grand Slam titles are two shy of Pete Sampras's all-time record, extended his US Open win streak to 31 matches by beating Russian Igor Andreev 6-7 (5/7), 7-6 (7/5), 6-3, 3-6, 6-3.

An ailing Djokovic had to dig equally deep while playing through a sore right hip to defeat Spain's Tommy Robredo 4-6, 6-2, 6-3, 5-7, 6-3.

"Novak got a tough one too. You always wish for three sets but honestly those five-setters are fun," Federer said.

Federer advanced to the quarter-finals where he will play Luxembourg qualifier Gilles Muller. Djokovic next faces the winner of a later match between American Andy Roddick and Chile's Fernando Gonzalez.

Federer, trying to salvage a sub-par season for him after years of Slam domination, hadn't dropped a set in his first three matches, but Andreev made him work for the victory.

It has been rare to see Federer in a five-set match at the US Open but this hasn't been a normal season for the former world number one. Close matches makes life more interesting and gets the competitive juices flowing, he said.

"I don't remember ever playing an entire five-set match on center court here. I played Agassi but that was over two days. I was always winning easily. It is different and fun to be in a dog fight," he said.

Federer had trouble getting untracked in the first set and it wasn't until he got his serve on track and was able to jump start his net game that he began to chip away at the hard-hitting Russian, who came into the US Open with a career-high No 23 ranking.

"He was playing well. He broke me so easily at the beginning and I was struggling to get in groove from the baseline. The second set was key for me to get back in the match. I served well but it was a tough match," Federer said.

Federer fired 11 aces and converted 69 percent of his net opportunities but he also served five double faults and made an uncharacteristic 60 unforced errors in the three-and-a-half-hour match on the Arthur Ashe Stadium.

The match started during the day session but ended up finishing well into the night session on Ashe, forcing organizers to move Jelena Jankovic's women's quarter-final to the smaller Louis Armstrong court.

Andreev gave Federer all he could handle in the fourth set as well. Federer survived three set points before sending a forehand long on the fourth set point to send the match to a deciding set.

But Federer rebounded in the fifth, saving four break points to take a commanding 5-2 lead and eventually the match.

With his normally reliable forehand stuttering and his body failing him, Serbian Djokovic also had to rally from an opening set loss to Robredo.

Djokovic, who now leads the career meetings with Robredo 4-1, raised both arms into the air as the last ball was struck and then pointed to his heart, knee and head as a crowd of 10,000 at the Arthur Ashe Stadium stood and cheered.

"I need to stay next to the net otherwise I will fall down," Djokovic said immediately after the match. "Really. It was an incredible match."

An exhausted Djokovic said he is still feeling the effects of his previous match.

"If I start talking about the things that are bothering me now, we can talk about this until tomorrow," Djokovic said. "Whoever I play next will be physically fitter than me that's for sure."

Robredo took a spill of his own in the fifth set, rolling near the baseline while lunging for a shot. Robredo accused Djokovic of exaggerating injuries to get some extra rest.

"If you are not fit enough then don't play. I have pain all over my body but I don't complain. Do I trust him? No. I think he took his time because he was more tired," Robredo said.

Luxembourg's Muller, meanwhile, became the first qualifier since 2000 to reach a Grand Slam quarterfinals by surprising Russian fifth seed Davydenko 6-4, 4-6, 6-3, 7-6 (12/10).