Federer found himself locked in a five-set struggle against the sort of player the world is accustomed to seeing him dismiss with ease, and it was only down the stretch that the four-time defending Open champion looked the part.

Avoiding as big an upset as tennis has seen in a while, Federer came back to beat 23rd-seeded Igor Andreev of Russia 6-7 (5), 7-6 (5), 6-3, 3-6, 6-3 Tuesday night to reach the quarterfinals at the year’s last Grand Slam tournament.

“Being down a set, and a tiebreak in the second set, there’s danger written all over that situation,” Federer said. “You just hope that it’s going to turn your way. It did.”

Third-seeded Novak Djokovic and 2003 champion Andy Roddick both won Tuesday to set up a quarterfinal duel. Djokovic pulled out a 4-6, 6-2, 6-3, 5-7, 6-3 win over No. 15 Tommy Robredo of Spain; Roddick routed Chile’s Fernando Gonzalez 6-2, 6-4, 6-1.

Djokovic, the 2007 U.S. Open runner-up to Federer and the reigning Australian Open champion, looked up in the stands and saw his mom pounding her chest repeatedly.

Djokovic responded in kind, bumping a closed fist over his heart four times, then using his right index finger to point there, point to each knee and point to his temple - looking up into the stands all the while.

“Just trying to show them, you know, how much effort I put into this match,” he said.

He needed every ounce of heart, smarts and energy he could muster, and he acknowledged that Roddick would be fitter.

“I’ve got to feel good,” Roddick said. “He’s got about 16 injuries right now.”

When Federer finished the match with a forehand winner, he shook his fists violently and yelled, then flashed a grin toward his girlfriend and others in his guest box.

Hard to recall the last time this guy was so pleased by a fourth-round victory. After all, Federer is a man who owns 12 Grand Slam titles, two short of Pete Sampras’ career mark. Who has won 31 consecutive U.S. Open matches. Who is trying to extend his record streak of 17 straight berths in Grand Slam semifinals.

And yet Federer couldn’t stop smiling at the end of the 3½-hour test, in part, he explained, because he found it fun to be pushed into a fifth set.

“I don’t give myself the opportunity that much, because I always win easily,” he said. “I was just really pleased with my fighting spirit.”

Djokovic was bothered at various points by his stomach, hip, right ankle and breathing problems.

“Somehow,” the No. 3 Djokovic said, “I managed.”

No. 2 Jelena Jankovic beat No. 29 Sybille Bammer 6-1, 6-4 and advanced to a women’s semifinal against No. 5 Elena Dementieva, who beat No. 15 Patty Schnyder 6-2, 6-3.

Federer’s close call surprised everyone, even if his 12 losses this year are more than he absorbed in any entire season from 2004-07.

Still, it’s one thing to lose to Rafael Nadal, the man who walloped Federer in the French Open final and edged him in a five-set thriller of a Wimbledon final. Nadal owns five Grand Slam titles and has overtaken Federer in the rankings after the Swiss star’s record 237-week stay at No. 1.

It would have been quite another thing to lose to Andreev, who entered Tuesday on a seven-match losing streak against top-five players.

“For me, it was great experience,” Andreev said.

He fell to 1-7 in five-set matches, and big-match toughness certainly was a factor at key stages.

That also could be the case when Federer plays 130th-ranked Gilles Muller in the quarterfinals Thursday. No. 5 Nikolay Davydenko 6-4, 4-6, 6-3, 7-6 (10) to become only the second qualifier to reach the U.S. Open quarterfinals.

Scurrying along the baseline to whip his go-for-it forehand and find a line, Andreev made the once-invincible Federer seem human - not just during points, but also between them.

Normally calm and collected, Federer often threw his head back in disappointment or screamed with delight.

But in the second set, Andreev accumulated seven break points - and Federer saved them all. And in the crucible of the fifth set, Andreev compiled four more break points, all with Federer serving at 4-2 - and, again, Federer handled the situation better, erasing every one.