With their anorexic levels of body fat, Tour de France cyclists are in constant danger of hypoglycaemia, the state when the body runs out of the sugar that powers the muscles. English cyclists call it "the bonk", the French call it "la fringale".

Yesterday Lance Armstrong came close to paying the price for not eating enough when he "bonked" on the final mountain of the race, the viciously steep Col du Joux-Plane, and looked vulnerable for the first time in this Tour.

"It was the hardest day of my life on the bike. I had no sugar, no energy," he said after losing one minute 37 seconds to Jan Ullrich of Germany, who finished second behind France's Richard Virenque, whose first stage win for three years quickened hearts across the nation.

Ullrich is now five minutes 37 seconds behind in second place. It remains a respectable cushion but Armstrong will be relieved that the only serious climb today is the long but evenly graded Col des Mosses in the heart of Gruyere country.

Devastatingly strong on every mountain stage hitherto, Armstrong was a different man in the pinewoods and high hayfields of the Joux-Plane, with its views of Mont Blanc and its vertiginous descent where each hairpin seemed tighter than the last.

He had begun the climb determined to gain the stage win that has eluded him. A searing pace was set by the little Spanish climber Roberto Heras, whose Kelme team had made much of the running in every mountain stage and who won the stages to Hautacam and Briancon.

Heras was the first to jump away, then Ullrich and Virenque slipped the leash. The clearest indication that the yellow jersey was in trouble came as the men who had been burnt off by the initial injection of pace made their way up to and past him.

"In a crisis like that it's better to stay in control, not panic and try to ride your own race," Armstrong said later. "I started the day with seven plus minutes lead and just tried to limit my losses. I knew I was in trouble but I just ate regularly and stayed conservative."

Marco Pantani was nowhere to be seen by then, but he had already made a point. He and his team had been angered by Armstrong's assertions the previous day that the Italian "lacked class" and that it had been a mistake to gift him the Mont Ventoux stage.

Pantani had a point to prove, and he zipped out of the bunch among the ski lifts of the Col des Saisies, the first climb of the day. The finish was 80 miles distant and he had no chance of winning the stage, but his 50 miles out in front wore out Armstrong's team-mates.

Pantani disappeared early on the Joux-Plane, leaving the field open for Virenque, another climber facing a drug-related court case in October.

The Colombian Santiago Botero has an impregnable lead in the King of the Mountains contest after his lone escape to Briancon on Saturday, and Virenque had no chance of winning a sixth polka-dot jersey.

A stage win was all that was left for Virenque to salvage from this Tour, and he took his chance when Heras misjudged a corner and piled into the crash barriers, leaving the Frenchman a clear run to the line for the fourth stage win of his career.