Can Federer hang with tennis' giants?

There was once a glorious time when Roger Federer sliced, diced and pureed virtually every opponent in his path.

From 2004-07, the Swiss star won 11 of 16 Grand Slam singles titles and his annual losses could be counted on just two hands. His exquisite gifts of timing and vision and mental toughness moved people to call him the greatest player of all time.

Recently, with his breathtaking game in relative remission, he's been overpowered by stronger adversaries. Two weeks ago in the Toronto final, Andy Murray -- famously passive in important matches -- tagged Federer 7-5, 7-5.

"He was aggressive," Federer said. "He was taking the ball earlier. Just all of a sudden it was all over."

The next week in Cincinnati, however, Federer was the last one standing. He beat Mardy Fish in the final and finished his summer U.S. Open tune-ups with a 9-1 record. He would appear to be the marginal favorite ahead of Rafael Nadal and Murray.

Federer's post-Wimbledon won-lost percentage (.900) was the best on tour, ahead of David Nalbandian (.846), Mardy Fish (.889) and Murray (.833).

Federer's artistry was at odds with the trend of bigger, faster, stronger athletes who dominated other sports. But now, it seems, tennis has joined that muscular arena.

"First we saw Big Babe tennis," said ESPN analyst Mary Carillo. "Now we have Big Boy tennis. I mean, let's face it: Roger's got to get physically stronger. This is the big time now."

Federer is listed at 6-foot-1, 187 pounds, but against the emerging crop of behemoths he looks smaller. Juan Martin del Potro, the strapping 6-6 Argentine, knocked him around in last year's U.S. Open final. Robin Soderling, 6-4, 192 pounds, pounded Federer in the French Open quarterfinals and Tomas Berdych, with an advantage of four inches and 13 pounds, did the same at Wimbledon. Federer won the Australian Open final with his third straight win over the 6-3 Scot, but the Toronto match suggested that dynamic might be changing.

Like Martina Hingis, who won three of the four majors in 1997 but slammed into the powerful wall that was the Williams sisters, Federer can no longer dominate without a game-changing weapon. Because he hits a one-handed backhand, he is ceding a significant margin in power to the rest of the truly elite field that uses two. And though Federer has the solid lower body of a man who makes his living on the run, his upper body -- particularly his arms -- is so lean it borders on frail.

"It is almost hilarious," Carillo said, "close to silly. What Roger's facing is a natural evolution. John McEnroe lost a step and guys who played more physically, like Becker, Lendl and Sampras, started to beat him."

Said Ivan Lendl, who at 50 will play his first match on the ATP Champions senior tour later this year, "He hasn't really worn out his body that much because of the style of his play. The issue is that the guys have gotten better. Berdych, del Potro, Soderling -- they all can serve bigger than Roger and hit bigger than Roger."

Recently, Federer has been working with Paul Annacone, Pete Sampras' former coach, in an effort to raise the level of his game. They spent time together in Toronto and are expected to collaborate again at the U.S. Open, which begins Monday in New York.

"The goal has always been for me to improve as a player," Federer said in Toronto. "I won't just be happy playing the same way for years and years and years. I've always questioned myself in the best of times and in the worst of times, even though there were not many bad moments the last seven years or so."

Nick Bollettieri, the legendary coach, has observed indecisiveness creeping into Federer's game. It can be seen, he says, in an increasing number of slices from the backhand side.

"When he hits that slice to Nadal's forehand, he's been able to flatten it out and send it back strong," Bollettieri said. "Sometimes when you go to the slice you're not as sure of yourself, especially when you have a one-handed backhand.

"Now, players want to play Roger -- that's not good. It's the message he's sending. He's got to believe; when did you ever see Roger put his shoulders down like at Wimbledon? Never. Champions have that fire, and Roger's got to rekindle that flame."

Federer says playing a lighter schedule keeps his intensity at a high level.

"I only play 16 to 20 tournaments per year, so I'm not overplaying, and that keeps the fire burning," Federer told the BBC a few weeks ago.

Darren Cahill, who coached both Andre Agassi and Lleyton Hewitt, believes Federer's issues are less about mentality and more about physicality.

"Losing in the quarterfinals probably set off some small alarm bells going off in the [Federer] camp," Cahill said. "Truth be told, every Grand Slam he plays, he's a factor. But the challenges are getting harder. He's going to have to work harder on the court -- and off the court."

In the recent past, Federer has tinkered with his game in run-up tournaments. Now, Cahill believes, Federer needs to raise his game in those smaller tournaments for more of a running start into the Grand Slam events.

"He needs to find his game quicker, build confidence quicker so when he gets to the second and third round of a major he's more confident, quicker," Cahill said. "Paul's influence will help him be technically smarter on court; knowing how to use strengths can physically cut some corners. Playing smarter in the earlier rounds will help him save some gas in the tank for later on."

Brad Gilbert, an ESPN analyst and former pro who coached Agassi, said Federer's recent decline is a natural evolution.

"In the last 24 months he's become more vulnerable in best-of-three sets -- until the French, that hadn't been the case in best of fives," Gilbert noted. "It's not surprising at all. Very few players have dominated tennis at the age of 29.

"Roger's played [43] majors in a row. The amazing thing about Roger is he's had one of the best runs ever of being healthy. In the final analysis, it's not about Cincinnatis or Romes -- the number everyone will be shooting for is 16, 18, 20 [majors]. It would be a major mistake to think that his last chapter has been written."

Added Lendl: "If you ask John [McEnroe] and myself about going out in the quarters of a Slam twice in a row -- the sky is falling," laughed Lendl. Maybe Tommy Haas or Tomas Berdych might think differently. The point is, Roger is head and shoulders ahead of everybody.

"What are his goals? Maybe to win as many majors as possible? Maybe to stand in Rafa's way of winning more [Slams]. Roger is not going to say `Well, I'm not playing that well. I'm playing for enjoyment. I'm done.' You're not going to hear that from him."

For some observers, Federer is a subject to be discussed in the past tense. But, to be fair, a win at the U.S. Open would give him eight of the 16 major titles available in the past four years.

Federer told the BBC that winning 20 major titles remained a realistic goal. That would be one each year for the next four years.

"Having won three Grand Slams per season three times and two per year a couple of times, it's something that's doable for me," he said.

Greg Garber is a senior writer for ESPN.com.