The iron will of Roger Federer [Images] and the burning desire of Novak Djokovic [Images] carried them into the quarter-finals of the US Open [Images] on Tuesday.

In a match that started under sunny skies but ended under floodlights, Russia's [Images] Igor Andreev exposed Federer's vulnerable side before the Swiss protected his four-year New York reign with a compelling 6-7, 7-6, 6-3, 3-6, 6-3 triumph.

"It was a tough match, the wind was changing with either side, it went from day to night session which is never easy. It was tough conditions but I'm happy we put on a good show," a relieved Federer summed up courtside after his 3-1/2 hour win.

Weakened by a dodgy stomach and stricken by a hip injury, Djokovic survived a physical meltdown to hobble past 15th-seeded Spaniard Tommy Robredo 4-6, 6-2, 6-3, 5-7, 6-3.

After watching Robredo net a forehand on match point, third seed Djokovic tilted his head skywards then locked eyes with his mother before thumping his heart four times with a clenched right fist.

He went on to point to his hip and forehead, as if to say: "My heart won me that one today."

Clutching the net with his left hand, an exhausted Djokovic said following the three hour 45 minute tussle: "I need to stay next to the net, otherwise I will fall down.

"If we start talking about the things that are bothering me now, we will talk until tomorrow. I am just really happy to get through."

Next up for the Serbian will be former champion Andy Roddick [Images], who dashed into the quarters by beating Chilean 11th seed Fernando Gonzalez 6-2, 6-4, 6-1.

Fifth seed Nikolay Davydenko's racquet felt the full force of his frustrations after he fell 6-4, 4-6, 6-3, 7-6 to qualifier Gilles Muller.

BROKEN RACQUETS

Davydenko was luckily spared the ordeal of contesting a fifth set because as the Russian said: "I broke all my rackets in the match and I didn't have any left for the fifth set."

Muller won a titanic fourth-set tiebreak 12-10 to become the first player from Luxembourg to reach the last eight of a grand slam and at 130 in the world, he also became the lowest ranked player to reach that stage here since 1999.

With the men hogging all the limelight, Olympic [Images] champion Elena Dementieva and second seed Jelena Jankovic were barely noticed as they tip-toed into the women's semi-finals.

Dementieva kept up her hot streak with a sizzling 6-2 6-3 win over Switzerland's [Images] Patty Schnyder.

With the trials of Djokovic and Federer lasting almost eight hours, Jankovic was shunted off the main showcourt but she took it in stride and eased to a 6-1, 6-4 win over Austrian Sybille Bammer.

There was nothing easy about Federer's win though.

From the word go, it was clear the second seed would be battling for survival when he meekly surrendered his serve in the opening game.

Armed with a deadly serve and an equally lethal forehand, Andreev had Federer running scared.

With a never-ending stream of errors flying off Federer's racket, an incredulous John McEnroe [Images] asked while commentating on the match: "Is this an imposter we are seeing?"

Unable to conjure his usual magic from the baseline, Federer shouted and yelled in frustration.

As the match headed into a fifth set, Federer had earned and converted only two break points while Andreev, seeded 23rd, had managed to squander nine of the 11 he had eked out.

Switching to serve and volley tactics, an unusually emotional Federer stamped his approval when he lobbed Andreev and saw the Russian chase the ball down to the baseline only to pull it wide.

It gave Federer a 2-0 cushion and with the 23,000 fans packed into Arthur Ashe Stadium roaring him on like a favourite son, the Swiss maestro fended off four more break points in the seventh game before sealing a last-eight date with Muller.

It was only the third time in 46 matches at the U.S. Open, and the first since 2004, that Federer had become embroiled in a five-setter.

"I'm really pleased with my fighting spirit. You always wish for three sets but those five setters are fun too... being part of some dogfights, it's fun for me," Federer said.

'Fun' was not the word Djokovic would use to describe his experience.

Barely 36 hours after surviving a four-hour marathon against Marin Cilic, the last thing Djokovic would have wanted was to be dragged into another tug-of-war battle on a hot and humid day.

The world number three had little say in the matter and despite his body letting him down, he would not give up.

Emitting an anguished groan every time he made contact with the ball, Djokovic's movements at times resembled that of a man carrying dead weights around his feet.

The 21-year-old received a rub down on his already-strapped right thigh, rolled his ankle and suffered from breathlessness but somehow found the energy to crawl over the finishing line.

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