When Lance Armstrong won his first Tour de France stage at Verdun in 1993 a journalist asked him where he was aiming for, given that his namesake Neil had made it to the moon. The young Texan, already turning heads with his cocksure personality and utterly focused racing style, modestly replied: "Mars".

Two and a half years ago, when Armstrong was diagnosed with testicular cancer which required immediate surgery, France, its Tour, the Champs Elysees and the yellow jersey might as well all have been on another planet. Yet tomorrow, in a sporting and medical miracle, he is set to ride into Paris as winner of the Tour de France. By any standards, even compared with men who have not had cancer, it will be one of the biggest Tour wins in recent years. Armstrong has defeated the time-trial and mountain specialists on their home terrain in crushing style and has exuded confidence, which has extended to his mainly American US Postal Service team. There is every chance that today he will extend his lead to the greatest margin since Bernard Hinault's 14 minutes 34 seconds in 1981. He already leads Fernando Escartin by a bigger margin than Miguel Indurain achieved in any of his five Tour wins.

The journey has been remarkable. No endurance athlete has ever recovered from cancer as advanced as Armstrong's, including surgery and chemotherapy, and returned to dominate an event as lengthy and demanding as the Tour. Once he left the Saint David hospital in Indianapolis, determined to continue his career as a cyclist, he was in uncharted waters.

"I'm totally confused. I'm going completely into the unknown. A comeback like this has never been attempted. I don't know, my oncologists don't know and obviously the sport doesn't know," he said when he set up his base in Nice at the end of 1997. Tellingly, he had no idea how much furniture to buy: he had no idea how long he would last in professional cycling. He had announced his cancer to the world in a telephone press conference at the start of October 1996; by the end of the month he was weak from chemotherapy, had lost his hair and bore the scars of surgery to remove lesions from his brain, abdomen and lungs.

"Initially, in the first two weeks, I thought I might die, but at the point where they discovered the lesions on my brain I was prepared to die." Still bitter Armstrong's comeback has always begged one question: why should anyone want to attempt to return to the most demanding endurance sport in the world when they have just cheated death? He has recalled the time he was diagnosed. "I was scared that I was going to die, and I was scared that I would lose my career. I'm not sure what I thought first: `I'll never race again' or `I'm going to die'. That's why it makes sense." There were other motivations: the fact that a successful comeback would prove to the cancer community that it was possible to combat the disease has been crucial to Armstrong. In addition, once he was healthy again he was seized with the desire to prove to a sceptical European cycling establishment that he could return to his previous level.

He is still bitter at the way he was treated by European teams during his illness. The Cofidis squad, who hired him just before his diagnosis, attempted to renegotiate his contract downwards as he lay in hospital. Other teams simply did not want to pay him anything like the salary he earned before he became an invalid.

"I was talking to Roger Legeay, the manager of Chris Boardman's team," Armstrong has recalled. "He told me what I was expecting was `the money of a big rider'. Teams think if I get sick again it will be bad publicity. There is no precedent for what I went through, maybe I'm naive or stupid, but I would have thought that people would want to be part of the story."

He is still bitter about the lack of belief in what he has achieved. On Wednesday, replying to the newspaper Le Monde's story on the fact that minute traces of a corticosteroid had been found in his urine, he said: "This is the story of someone who was not given any chance, everyone said I could not come back. I saw the same mentality when I wanted to find a team, and no one wanted me because they said it was not possible." There have been false starts in the past two years as Armstrong became "completely terrified that the illness was coming back". In March he left the Paris-Nice stage race and was on the point of quitting. Several weeks of monastic seclusion in North Carolina prompted another rethink, and that marked the start of the upward trajectory which has taken him to the verge of victory in this Tour.

The principal pointer towards his current performance was Armstrong's fourth place in the Tour of Spain in September last year, after which he reflected that even racing in the Madrid sierra in hail and snow was nothing like as bad as being treated for cancer. He had never climbed mountains with the ease he showed then, but during his illness his physique had changed, with the loss of the broad swimmer's shoulders which were the product of triathlons he raced in his youth.

He has another explanation: "I was half dead and was put back together by the best doctors in the world. Perhaps the illness was there for a while and I was training and living with it. Perhaps when I got rid of it, that helped me. You can imagine if you have an advanced form of cancer what it does to your body."

He has also pointed out: "To race and suffer, that's hard. But that's not being laid out in a hospital bed in Indianapolis with a catheter hanging out of my chest, with platinum pumping into my veins, throwing up for 24 hours straight for five days, taking a two-week break and doing it again. We've all heard the expression `what doesn't kill you makes you stronger' and that's exactly it."

Perhaps the same will be true of the world's greatest bike race. Even after the drug scandals, police inquiries and revelations of the past year, the Tour de France still inspires awe at the physical and mental efforts it demands of its participants.

Its attraction for its international crowds which, on the evidence of this year's roadsides, are still as warm as ever in their affections, has always been founded on the fact that most people ride a bike and can have some idea of the physical effort involved. One thing sets this year's race apart from the 85 that have preceded it: only Armstrong and the rest of the cancer community truly know what it has taken to win.