Cycling Tour de France: William Fotheringham on how the leader's fragility has guaranteed an epic Tour finish

There is a point in the career of almost every giant of the Tour de France when, as the French put it, "the wheel turns": anno domini catches up and decline sets in. "You don't see it coming but there is a time when it just becomes more and more difficult to win," says the double Tour winner, Laurent Fignon.

Fignon won the 1983 and 1984 Tours with insouciant ease, dominating the 1984 race with such power he was temporarily nicknamed The Ogre. At the end of his career in the early 1990s, however, it all changed.

"Cycling isn't an exact thing but suddenly you begin to doubt yourself. You decide what you want to do in a race and you can't do it. You make little excuses, you say you weren't feeling good or this or that. There are lots of races when things are no longer as clear-cut as they were. It's frustrating, it's annoying, and some cyclists stop soon after they get to that stage, although some keep going because they like riding their bikes. I stopped, because it's simply not fun. It's very hard to go on for years like that."

Only one of the cyclists who have won five Tours has managed to get out while still at his best: Bernard Hinault, who looked as strong in his last Tour, 1986, as he did in his first. All the others - Jacques Anquetil, Eddy Merckx, Miguel Indurain - went downhill before retirement, for varying lengths of time.

Even if Lance Armstrong wins the Tour de France by defending his maillot jaune in the time-trial stage through Brittany today, as on paper he should do, there is bound to be speculation that his fifth Tour win may turn out to be his last, because he has displayed signs of weakness for the first time since his return from cancer.

Whatever the reasons - problems in his marriage, the hot weather, the emergence of younger rivals - the Texan has not dominated this Tour as he did the previous four. If he holds off Jan Ullrich today this will probably end up as the most impressive of his sequence of wins, purely for the way that he has overcome adversity and kept a cool head when all around him are falling off their bikes, but the signs of decline are still there.

Armstrong turned up at the start of the prologue time-trial in Paris without having looked at the course, a rare lapse for this most meticulous of cyclists in a race where every second counts. In the Alps he looked on as Alexandre Vinokourov and Iban Mayo attacked. In the blazing heat at Cap Decouverte he lost an unheard-of 95 seconds to Jan Ullrich in the time-trial stage. In the first Pyrenean stage, at Ax Trois Domaines, he talked of "crisis" and "disaster" and admitted "something was not clicking".

The Armstrong of old has been seen once in this Tour, on the third day in the Pyrenees, when adrenalin fired him to his only stage win of the race. It is to that he owes the bulk of his lead of just over a minute on Ullrich.

There is, says the Tour's official historian Jacques Augendre, a clear trend for the Tour's greats to struggle to win their fifth Tour.

"Jacques Anquetil only won his fifth, in 1964, because Raymond Poulidor punctured in a time trial. Perhaps it is a wearing-down process, because the Tour is simply so hard, and the proof is that so far no one has won six. Even Merckx and Indurain flopped when they went for six."

The signs of decline are visible even in one of the Tour's greats, says Augendre. "The cyclist's style becomes more laboured. His attacks have less vigour. He controls the race a little less, by waiting and thinking before reacting to something rather than reacting immediately."

Fignon, however, is not convinced Armstrong is fading just yet. "I'm not sure this is decline. Perhaps he's not quite as strong but the point is you can't just get stronger every year. At his age, when you are no longer getting stronger physically every year, you can keep on the same level only if everything goes well and if you work harder each year.

"He suffered in the heat in the middle 10 days of the Tour and he didn't open his usual gaps on the others even when he did escape at Luz Ardiden, but the other riders went faster than before. Only Armstrong knows how he is compared to past years and he isn't going to tell us. All that matters to him is whether he wins today."

The centenary Tour has captured hearts and minds across France. Yesterday, the crowds coming through the little-visited Deux-Sevres region were as large as they usually are in the cycling heartland of Brittany. This race will be remembered as one of the great Tours, largely because Armstrong's new air of fragility has guaranteed suspense right up to the race's final weekend for the first time in 14 years.