Everyone has a theory about Roger Federer. He doesn’t volley enough. He needs to run around his backhand more often. He’s lost his confidence. He’s tired. He still hasn’t recovered from mononucleosis. He needs to spend more time with his part-time coach, Jose Higueras. He should take the rest of the year off (after the U.S. Open) and practice. He needs a sports psychiatrist.

The latest theory is that Federer has spent too much time of late worrying about his image rather than his strokes. He agrees to more photo shoots, plays more exhibitions, and travels to more ancillary events such as the Super Bowl. He spends more time hobnobbing with folks such as Anna Wintour. Can IMG, Federer’s marketing agency, be blamed for taking Federer’s eyes off the ball, for the demise of the world’s best player?

As you’ve probably guessed by now, I’m not convinced by any of these theories or excuses. We’re not talking about an impressionable underage gymnast here, but one of the most independent and determined — sometimes to the point of being stubborn — no. 1 players in the history of tennis, the same man everyone was ready to anoint the greatest player in history (and perhaps deservedly so) less than a year ago, when he won his fourth consecutive U.S. Open and 12th major title.

Federer does things his way. When he was a temperamental, racket-smashing teenager, coaches asked him to change. He did — eventually, slowly, and on his own terms. He played for years without a coach, but you’ll recall that when he most seemed to need one — leading up to last year’s French Open, when he lost to the less-than-fabulous Filippo Volandri — he parted ways with Tony Roche. Federer promptly beat back the impending doom, reached his second consecutive French Open final, and a won a thrilling Wimbledon final against Rafael Nadal in five sets. He topped off the year with his most convincing victory over Nadal to date, at the year-end Masters Cup in Shanghai.

Federer did a lot to build his image, and fatten his bank account, last year and the two years before that. He also won eight major titles, so no one said a word. He didn’t have to fly around Asia last November with Pete Sampras, or meet Sampras again in March in New York. But he chose to do it. If there’s one flaw in Federer, maybe it’s that he became convinced that he could do everything — play a full season, play exhibitions, pitch products for Gillette and Rolex — and not tire himself out. But who with a résumé like Federer’s wouldn’t have been convinced of this? For Federer, next to nothing went wrong in his life — on the tennis court or off of it — for four years. Wouldn’t you feel invincible? Without that bit of bad luck earlier in the year, when he contracted mono, Federer might now be pressing ahead with another Wimbledon trophy and not a peep from the skeptics.

When I look at Federer now, I don’t see a man in decline. I see a champion rattled, an all-time great player facing his first moment of crisis; a man confronted, for the first time in his life, with someone who is, at this moment in history, superior. For more than four years, Federer set the standard in tennis. In all those years, though, Rafael Nadal watched and learned and practiced harder and harder. Slowly but surely, he caught up and did what no one else had been able to do. Federer, finally faced with a man whose talents are as prodigious as his own, now must respond.

Bjorn Borg’s 1981 loss to John McEnroe in the Wimbledon final is often seen as the end for Borg, who quit tennis the next year. Many forget, though, that Borg had a chance for redemption at the U.S. Open in the summer of 1981. Borg had never won the tournament; the year before, he trailed McEnroe two sets to love, won sets three and four, but lost in the fifth. In 1981, he won the first set, but lost the next three. He never played another major tournament.

Borg couldn’t cope with those defeats. But other champions have. Sampras won another major title after Federer defeated him at Wimbledon in 2001. Andre Agassi suffered a demoralizing defeat to Sampras at the 2002 U.S. Open, but recovered to win his fourth Australian Open, and final major title, in 2003. Jimmy Connors dominated the tour in 1974, winning all three major tournaments he played (he skipped the French Open). By the end of 1981, he hadn’t reached a major final in three years and many people thought he was finished. Then he won two major titles in 1982 and another one in 1983.

What Federer does next might define his place in this sport more than anything he has done up to this point. There’s no chance he’ll end his career this year, as Borg did, even if he doesn’t win his fifth consecutive U.S. Open two weeks from now. That’s just not Federer’s character. But there is a chance he won’t win another major title, if for no other reason than the ages of Nadal, 22, and Novak Djokovic, 21, who both could improve in the next year or two, assuming they remain healthy. Of course, Federer might yet win four more majors. No doubt, he is looking ahead at the most difficult and uncertain years of his career, a period in which belief and self-doubt will do battle from tournament to tournament. Tonight, at Arthur Ashe Stadium, the journey begins.

Mr. Perrotta is a senior editor at Tennis magazine. He can be reached at tperrotta@tennismagazine.com.