Last year, Roger Federer strolled into Wimbledon in long trews and white jacket. He hung the jacket carefully on the back of his chair and performed the warm-up for his first match in the trousers. He looked like a glorious throwback to a more gracious age: serene, calm, quietly certain of his own genius. Life should be like that.

I called The Times Chief Sports Photographer, Marc Aspland, to ask him, please, please get some trouser shots. His phone was switched off, as etiquette requires, but he knows a picture when he sees one. My “welcome Rodge” piece made the paper with the man in his full trousered glory. It was obvious from that first warm-up that nothing could stop him, and nothing did.

Now, 12 months on, we may get the jacket again, and we may get the trousers as well, but we certainly won't get the serenity or the self-certainty. Both have been lost in the wash. Federer is a player in decline. He went out of the Australian Open in the semi-finals, then suffered defeats by Andy Murray and Mardy Fish, allowed Andy Roddick to beat him for the first time in five years and then had three defeats in finals to Rafael Nadal. In the last of these, in the French Open in Paris, he won only four games.

Something's bloody well wrong.

But here's a little-known fact about champions: things go wrong for them, just as they go wrong for you and me. The great champions aren't the ones for whom nothing ever goes wrong. The great champions are those who deal with it.

In 2002, Pete Sampras sat on his chair on No2 Court reading and re-reading a touching letter from his wife, Bridgette. It said all the things that your beloved tells you when you are down on your luck and yourself. It was addressed: “To my husband, seven times Wimbledon champion.” It didn't work. Sampras was beaten in the second round by George Bastl, of Switzerland, a clay-courter ranked No145 in the world. Sampras was left a broken man. He hadn't won a tournament for two years, not since Wimbledon 2000. That, then, was the end of Pete.

Except it wasn't. He went on to the US Open, where Greg Rusedski said he was “a step and a half slower”. But so what? Sampras beat Andre Agassi in the final for his fourteenth grand-slam title. It was the last match he played on the professional circuit.

So I would like Rodge to look on this as a letter of motivation. It doesn't have the love of Bridgette, or, for that matter, of a Mirka, Federer's companion of choice. Still, it has the admiration of someone who has seen a few champions, and seen a few champions fall as well.

And I have learnt that the great champions are the ones that rise again. We don't need to look for obscure names here: how about Muhammad Ali, Pelé and Sir Steve Redgrave? In 1967, Ali was stripped of his world heavyweight title and barred from boxing. After three years, he was allowed back. After a series of small victories he got a title fight. “It's gonna amaze yer when I beat Joe Frazier.” But he didn't. He lost the “fight of the century”. But there was a better fight coming.

Although not before more setbacks. Ali had his jaw broken by Ken Norton, before lining up a title bout with the new champion, George Foreman. This, of course, was the Rumble in the Jungle. Foreman had destroyed Frazier, no one gave Ali a chance. But Ali won, in sensational style.

Pelé had a career that seemed blessed throughout; but it wasn't. He suffered a ferocious setback in 1966, at a World Cup that is usually remembered for other things. But it was a clogfest, that World Cup, and Pelé was scientifically kicked out of it. He was brutalised against Bulgaria, missed the match against Hungary and was carried off against Portugal. Brazil went out in the group stage and Pelé said he would never play in a World Cup again.

But he did. He played in 1970 and Brazil won with what is remembered as the greatest team who ever played, with, naturally, Pelé at the heart of them. That's the sort of thing champions do.

So let's have Redgrave: four Olympic gold medals, the announcement that anyone who saw him in a boat again could shoot him, then the decision to come back for just one more. But the next year, now aged 35, he went down with diabetes. This is a hard condition for anyone to live with, for an elite athlete, it is challenging beyond belief.

Redgrave tried a low-sugar diet, but that didn't give him enough fuel. So he went back to a high-sugar diet and regulated the insulin accordingly. Most diabetics test their blood daily; Redgrave tested his ten times every day. He went to the Olympic Games in Sydney with a four that had tasted bitter defeat. Many said they couldn't win. But they did. And Redgrave has five gold medals.

So, if I still have Federer's attention here, there is a lesson to be learnt. Champions do not get it all their own way. Champions cannot rely for ever on their reputations. Champions find that there are times when self-certainty is taken from them, and that's when we find out what they're really like.

The first thing that is required, of course, is the acknowledgement that there is a problem. When you're slurping soup through a straw, when you're flat on your back on a stretcher, when your life depends on a jab of the needle, the existence of a problem is hard to avoid.

But so far, Federer's response to his reverses has been denial, at least in public. His clay-court season was satisfactory, Australia was perfectly acceptable. There's nothing wrong that a little grass won't improve; I once held the same belief myself.

It seems that Federer is operating on the principle that Wimbledon will make everything all right. But I've done more Wimbledons than Federer. I know that while this place is supremely good to its winners, it is always, in the nicest, kindest and politest possible way, unbelievably cruel to its fallen champions. Ask Sampras. Ask any of the greats.

Wimbledon is the best place to win, but it's the worst place to lose. There is something peculiarly final about defeat there.

Federer, at 26, goes into Wimbledon looking beatable. His year has been marked by an extraordinary - for him - level of unforced errors. Players will go against him fancying their chances. Novak Djokovic and Nadal potentially stand in his way.

We will know a great deal more about Federer in 2 weeks, and perhaps over the next couple of years. The moral is that if Federer wants to be mentioned in the same breath as Pelé, Ali, Redgrave and Sampras, he'd better get down and dirty, dig into the sordid and slightly disgusting parts of himself, things that he hasn't needed in years, and come out scrapping and snarling. Can Roger still snarl, do you think?