The 37-year-old American, back on the saddle after 3-1/2 years in retirement, is a fraction of a second behind overall leader Fabian Cancellara and heads Astana colleague Contador by 19 seconds.

"I think it's very possible that he (Contador) sets a tempo nobody can follow, and in that case there is not much I can do," Armstrong told reporters.

"I have to stay with the other favourites, and fulfil my responsibility as a team mate. I can't chase him down. If he goes alone, then I will just stay with the other favourites."

Spaniard Contador, Tour winner in 2007, will be in his element in the mountains and the scenario of the race will largely depend on his tactics in the final 10.6-km climb to Arcalis at an average gradient of 7.1 percent.

Should Contador attack Armstrong, he is likely to claim the yellow jersey from Cancellara and establish himself as a real leader within Astana but it would put the pressure on him and his team mates for two weeks.

"I think Lance does not have the physical means to beat Contador in the mountains," Charly Mottet, who was fourth in the Tour in 1987 and 1991, told Reuters on Thursday.

"If I am Contador, I don't take the (yellow) jersey, it would be like committing hara-kiri.

"I don't think he will try and get the jersey. It would be a serious tactical mistake. Armstrong in yellow would be the perfect mule for Contador," Mottet added.

Leaving the yellow jersey to Armstrong by staying with him until the line would put Contador in a perfect position as he is likely to be the strongest man in the mountains after three weeks.

He would have to wait before making his final move, though.

"If they arrive together at Arcalis, Lance Armstrong will have the yellow jersey and Alberto will be in second position," said Astana sports director Alain Gallopin.

"Look at Bernard Hinault (1985 winner) and Greg LeMond (1986 champion). They never attacked each other, even if there was a rivalry between them.

"Alberto is a leader but Lance is not and will never be a rider like others," he added.

Other favourites, who lost time in the earlier stages, will be forced to go out all guns blazing in the ascent to Arcalis.