I have been looking forward to the fallibility of Roger Federer for a long time, ever since I first saw him reach perfection at Wimbledon six years ago. Please don't think I have anything against perfection, but when you have watched as much sport as I have, you look for something more.

Federer has won 12 grand-slam tournament titles, been No1 since February 2004 and dominated his sport with style, grace, elegance, venom, accuracy, tactical brilliance, glorious angles, extraordinary reactions and a forehand bafflingly described as “a liquid whip”.

Many say that Federer is the greatest tennis player to pick up a racket. Most experts disagree only on the subject of how far he will outdistance the records of lesser players. But now, Federer is at crisis, and I am enthralled.

This is not because I wish bad things on him. This is not what Australians call Tall Poppy Syndrome: the ever-present need to cut exceptional people down to the same level as the rest of us. I am fascinated by giants, I have spent most of my professional life in pursuit of giants and I have learnt one or two things about giants as a result.

The core question is this: what does a giant do about fallibility? Federer, like many supreme performers, was able to add the liquid whip of invincibility to his armoury. He was able to beat people by reputation, by means of his serene certainty of victory. That is as good as a break start in each set.

But now that weapon has been taken from him. He lost in the semi-finals of the Australian Open this year, ending a run of ten consecutive grand-slam tournament finals. Then, still worse, he lost his next match, going out in the first round in the Barclays Dubai Championships this week to Andy Murray.

Federer can no longer rely on invincibility. He can no longer, and this is a still bigger blow, rely on mere excellence. Flawed performances have opened the door; now every player in the top ten is aware of his weaknesses and will seek to exploit them.

This could be the end, or perhaps not. Many sporting champions have been stripped of their invincibility and the physical talents that set them apart. Where do you go from there? Bjorn Borg, in a similar situation, chose to quit. He left tennis at the age of 26.

Pete Sampras won 13 grand-slam tournament titles before it all went wrong. But then he returned to Wimbledon - as champion, naturally - and lost in the fourth round to a promising upstart named Federer. The next year, he fell apart. He lost a second-round match at Wimbledon to the immortal George Bastl, in a tournament he had won seven times, at a place he was unbeatable. The sight of a great champion reading and re-reading a motivational letter from his wife was one of the most poignant images of the fallen champion in the history of sport.

But a few months later, Sampras summoned up one utterly glorious last hurrah. He won the US Open, beating Andre Agassi in the final, and it was a crystal-clear, irrefutable demonstration of the principle that great champions can win great matches even when stripped of their best skills and cloak of invincibility. Great champions have something more than mere ability.

It happens in all sports. Let us turn to Sebastian Coe. That's right, Lord Coe, the smooth-tongued diplomat, the slim fellow with the suit, one of the most effective politicians in the history of British sport. I know his secret. The man is a berserker. He has a temper like Krakatoa, and it is built on an inner anger of molten rock. He has an aggression that makes Vinnie Jones look like the amateur he is.

I know this because I watched the 1,500metres final of the 1984 Olympic Games in Los Angeles. Four years previously, Coe won the 1,500metres in Moscow on sheer gazelle-like talent. It was the occasion when he gave his talent full rein, and it was glorious to behold.

After four years in which injury ate away at his performances and the widespread belief that we had seen the best of him niggled at his soul, he went into the Olympic 1,500metres final. In three and a bit laps, anger, aggression and resentment turned him into the Human Torch. Unable to control himself, he celebrated by roaring at the press box: “Who says I'm f***ing finished?” He stabbed his finger upwards so often and so hard that I feared he might do the sky an injury. He, too, found something more than talent.

It can happen in team games. Jeff Thomson and Dennis Lillee were the finest fast bowlers on the planet. Working in harness, they terrorised the batsmen of the world and changed the way cricket is played. As they got older, Thomson - perhaps the better, and certainly the more fearsome - was unable to adjust. But Lillee cut down his pace and expenditure of effort and became a bowler of brilliant intelligence. He had not only talent, but also the talent to adapt.

Steve Redgrave and Matthew Pinsent, the twin knights of rowing, were each required to find something more than talent: and both did so. Redgrave, famously unretiring for a tilt at a fifth gold medal, met setback after setback, including the onset of diabetes. He was no longer the dominant force in his sport, but by adapting from the pair to the four, by ceding much of the moral leadership to Pinsent, and, above all, by his own phenomenal desire, he forced out one last success.

Four years on, Pinsent was in a crew that fell apart, endured every kind of strife and finally, at the Olympic Games, seemed beaten ten strokes from the finish. That final outpouring of will from Pinsent was one of the most vivid passages of sport I have seen. When you take everything from a champion, this is what is left.

But let us have one more, and perhaps the greatest. What did Muhammad Ali do when he no longer had the agility, the aura of invincibility, the quickness of hand, the lightness of foot? What was his response when floating and stinging were beyond him? What did he do, when he faced one of the greatest punchers in boxing history, the unbeatable George Foreman?

That was the Rumble in the Jungle, the fight in Zaire in which Ali showed that he had courage, patience, the brilliance and the strength of will to beat Foreman with his rope-a-dope tactic. It was a night that proved, once again, that when you take everything away from a champion - the skills that made him great and the aura that allowed him to stay that way - you can still find something more.

I don't know exactly what it is, and nor does anybody else, but by God, I know it when I see it. It is nothing less than the quintessence of greatness, and I long to see it again. Perhaps at Wimbledon this summer.