ESPN 2008 US Open

NEW YORK -- The percentages were stacked in his favor, in the manner to which he was long accustomed. Going into Thursday's U.S. Open quarterfinal, four-time defending champion Roger Federer hadn't lost to a lefty other than Rafael Nadal in more than five years. He hadn't lost to a player ranked outside the top 100 in more than three years. He'd won three of his previous four matches in straight sets.

All that and more put Federer's 7-6 (5), 6-4, 7-6 (5) win over strapping southpaw Gilles Muller of Luxembourg, a qualifier ranked No. 130 in the world, squarely in the realm of the expected. Yet there's still a sense that Federer is battling for a sort of come-from-ahead victory.

He hasn't won a Grand Slam event this year, and it's the first time in 19 majors that he hasn't come in as the top seed. That demotion earned him a sympathy cheer from traditionally unsympathetic New Yorkers during a ceremonial appearance on opening night. Perhaps just as tellingly, Federer hasn't even reached a hard-court final in 2008.

This has been Federer's year of living defensively. He has resisted the notions that he's still scrambling out of the sinkhole created by his winter bout of mononucleosis or that his confidence has been dinged and can't be hammered out. Yet despite his protestations, there's no doubt that winning here would be monumental even for a guy whose career is strewn with landmarks.

He stonewalled a bit on the point. "Speculation, right?" he said. "So it depends on the moment, after match point, how I feel right then, so I can't answer that right now. I mean, I wish I could talk about it, how great it was winning the U.S. Open, but I'm not there yet."

Although Federer might not want to hypothesize, ESPN commentator and Davis Cup captain Patrick McEnroe said that the stakes are high and that Federer is responding with greater focus.

"I don't think he's playing quite at the level that he was in the last couple years, but he's certainly capable of turning it around," McEnroe said. "So I do think that this event for him is big, not just for this year and to win one major, but as far as moving forward to next year and maybe his confidence coming out in '09."

A U.S. Open title would give Federer 13 Slams, just one short of Pete Sampras' record 14 -- a mark that, when the 2008 season began, it seemed inevitable Federer would reach if not surpass. "It's not going to get any easier for him, I think, in the next few years," McEnroe said.

It wasn't all that easy Thursday, as Muller pushed Federer to two tiebreakers on a sticky, breezy day. Six-foot-5 Muller, who won his own personal Slam by marching through three qualifying matches before winning four in the main draw, displayed an impressive serve-and-volley game that made one wonder why he hasn't had better success on the ATP Tour.

The 25-year-old has been laboring on the lower-level Challenger circuit for most of the season. He won the 2001 junior U.S. Open and is best remembered in Flushing Meadows for derailing Andy Roddick's mojo with a first-round upset in 2005 -- the same year he reached a career apex of No. 59.

Muller staved off five set points serving at 5-6 in the first, but succumbed in a tiebreaker highlighted by a 22-shot rally on his serve that ended when he dove in vain for a whippy Federer backhand down the line. Federer earned his only break of the match to go up 5-4 in the second, and prevailed in the second tiebreaker despite having trailed 4-1.

As he has all tournament, Federer vented his emotions on big points and let out a few vehement war whoops. "Who knows, maybe it is just a couple of weeks and I'm going crazy, and after that you'll see me more relaxed again," he said.

Winning the Olympic gold medal in doubles may have helped take the edge off Federer's competitive frustration, but it's safe to say there's still a considerable backlog. Rather than mourn all the streaks that have ended this season, he chose to celebrate the one that's still alive -- his 18 straight appearances in a Slam semifinal.

Still, someone with standards as high as Federer's isn't interested in consolation prizes. "I hope this time around I can take it a step further than I did in Paris or Wimbledon," he said. That's just the tip of an iceberg that he hopes will sink his next opponent.

Bonnie D. Ford covers tennis and Olympic sports for ESPN.com. She can be reached at bonniedford@aol.com.