The 37-year-old Armstrong has impressed many observers with his performance after being away from the sport for nearly four years. But he has looked uncharacteristically vulnerable in the mountains and could lose much time on Mont Ventoux.

His thoughts may already be on next year's Tour de France, in which he will be competing with a new team.

Late Thursday he announced that the US electronics retail giant Radio Shack would be sponsoring his new team in 2010. It is widely assumed that Johan Bruyneel, the manager of Armstrong's current team, Astana, would be joining him in the new venture.

Bruyneel directed all seven of Armstrong's Tour titles and has already said he would be leaving Astana by the end of the year.

The Tour ends Sunday in Paris.