It's but one measure of Roger Federer's grace on the tennis court that 12 years into his pro career he hasn't suffered appreciable injury or endured an extended layoff.

Ease of movement and uncommonly fluid strokes are hallmarks of the Swiss champion's game - considered the most complete and aesthetically pleasing in the sport.

But as the 29-year-old Federer launches into pursuit of what would be his sixth U.S. Open title when the season's final major gets under way Monday, it's an open question whether sheer majesty will be enough against the sport's new breed of power hitters and massive servers.

Federer, the tournament's second seed behind world No. 1 Rafael Nadal, faces a possible quarterfinal against one of those full-tilt blasters, Robin Soderling of Sweden, who ushered him out of the French Open earlier this year.

Should the Swiss prevail, a semifinal against third-seeded Novak Djokovic or big-serving Andy Roddick, who has reclaimed his place among the top 10 after a bout of mononucleosis, likely awaits.

Regardless of whether he reaches a seventh consecutive U.S. Open final, the proceedings at New York's Billie Jean King Tennis Center may represent a crossroads for Federer, who has ruled the sport for so long that the only place to go, it appears, is down.

If Federer has designs on reclaiming his No. 1 ranking and padding his record 16 major titles, as he has intimated, he may well have to add more punch to his strokes and gamble more on his return of serve, which can be a bit passive.

All that is a far cry from saying Federer "is done," as pundits periodically crow.

"The guy is the youngest 29-year-old I've ever seen," gushes veteran coach Brad Gilbert, an ESPN analyst. "He hardly ever sweats; he's hardly ever injured. He's incredibly youthful, so writing him off is a mistake."

But Federer is vulnerable in a way he hasn't been in recent years.

It's not so much that he's "lost" anything. Even experts struggle to pinpoint a tactical weakness in his game.

What has closed the gap between Federer and Nadal, who holds a 14-7 record against the Swiss, and Federer and a handful of elite contenders such as Andy Murray, who boasts a 7-5 edge?

A younger generation of bigger, harder hitting challengers is remaking the game's dynamics. Among them: Nadal, Soderling, Tomas Berdych and Juan Martin del Potro, who snapped Federer's streak of five consecutive U.S. Open titles last year in a brilliantly played five-set final.

Moreover, Federer's aura of invincibility has been pierced as a result of some high-profile losses, such as the evisceration by Nadal in the 2008 French Open final, followed by the five-sets defeat to Nadal weeks later at Wimbledon.

While Federer came out roaring this season, spanking Murray in straight sets to win the Australian Open, he fell to the young Scot earlier this month, 7-5, 7-5, at Toronto.

"The biggest thing is the fear factor in the tunnel," Gilbert says. "Guys go out feeling like they have a much better chance against him than they've had in the past."

And it's true. From 2004 to 2006, Federer was all but unbeatable, posting a staggering 247-15 record in matches and winning eight of the 12 majors contested.

So is it time to tinker with greatness?

Gilbert believes so, at least at the margins.

"When you get older, basically you're trying to stop the slide," Gilbert said. "You've got to add. In any sport, unfortunately, if you don't add, the competition catches up to you."

By all accounts Federer arrives in New York in peak health and form, having rebounded from the loss to Murray in Toronto by winning the title in Cincinnati.

Asked afterward if he felt a younger generation closing, Federerer said that after 12 years on the circuit, he was well accustomed to challenges-from veterans, youngsters and peers alike.

"I used to play against [Pete] Sampras and [Andre] Agassi, and [Carlos] Moya and Todd Martin and all those guys. Next generation was sort of already my generation almost: It was [Andy] Roddick and [Lleyton] Hewitt and [Marat] Safin and [Juan Carlos] Ferrero.

"So I've played through many generations now. Always been different challenges and rivals. That's not gonna change. I think every five years you'll have someone new. I think the next generation is probably already sort of knocking on the door."

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