The four-time champion held the overall lead by a handful of seconds following Tuesday's 10th stage. He is by no means clear of the chasing pack just a few days before Friday's individual time trials.
"It may be the most important time trial I've ever done," said Armstrong, who finished 45th Tuesday and is going for his record-tying fifth straight Tour victory.
"I've been focusing a lot on the time trial this year in terms of training," Armstrong said. "I know the course pretty well."
Showing signs of vulnerability in this year's centennial Tour - Armstrong was seriously challenged in the recent Alpine stages - he needs a good time trial more than usual. After Friday, Armstrong faces four grueling stages in the Pyrenees.
A good performance in Friday's 29-mile race against the clock would give him a crucial time advantage to take into the mountains.
"Lance usually makes a strong showing right from the start in the Alps, as if to say to his rivals, `You've got to come and beat me,"' said Stephen Roche, a Tour winner in 1987.
"But the fact he didn't means he couldn't do it. Don't forget - Armstrong is a year older now," Roche added.
Armstrong is seeking to tie Spain's Miguel Indurain as the only cyclist ever to win cycling's showcase event five times in a row.
Three others, Frenchmen Bernard Hinault and Jacques Anquetil and Belgium's Eddy Merckx, have also won five Tours - but none did so consecutively.
With a rest day Wednesday to recover from a strenuous first half of the Tour - so far Armstrong has crashed, had technical difficulties with his bike and problems with his racing shoes - the 31-year-old heads into Thursday's 11th stage just 21 seconds ahead of Kazakstan's Alexandre Vinokourov overall.
Spain's Iban Mayo is only 1 minute, 2 seconds behind Armstrong, while Germany's Jan Ullrich - reputed as a rider who gets stronger in the second half of the race - is 2:10 back in sixth place overall.
"Vinokourov is looking really good this year and I think he can win it," said Australian rider Baden Cooke.
In Tuesday's 10th stage, Armstrong finished in a large pack of riders who completed the 136-mile stage from Gap to the southern port city of Marseille way behind the winner, Denmark's Jakob Piil of Team CSC.
However, all of Armstrong's key rivals finished with him, meaning they did not gain time on the champion.
With temperatures on the route hitting 104 degrees, Armstrong said he suffered in the unrelenting heat.
"It's definitely the hottest Tour that most of us can remember," he said. "We've always had hot days but never so many in a row."
Wearing the leader's yellow jersey, Armstrong finished 21:23 behind Piil - who had never won a stage at the Tour.
Piil was among the breakaway group of nine riders surging ahead of the main pack just 9.9 miles into the race. Their breakaway lasted more than 124 miles - the longest of this year's Tour.
Piil beat Italian rider Fabio Sacchi of the Saeco team in a final sprint at the finish. Bram de Groot of the Netherlands, with the Rabobank team, was third.
"I've been chasing a stage victory on the Tour for two years now," Piil said. "I finally have it."
The race was marked by a protest that forced the chasing pack of riders to a sudden halt after supporters of a jailed farmer, Jose Bove, ran onto the road near Pourrieres, about 91 miles into the race. Bove is serving jail time for destroying genetically modified crops.
Tour officials immediately ruled that the protest was "a normal race incident," meaning that the riders who lost time because of the protest would not get it back.
Of the original field of 198 cyclists vying for Tour victory, only 141 riders started Tuesday. The most prominent rider to drop out was Joseba Beloki, who sustained multiple fractures in a downhill crash Monday. Before the accident, he was considered a major threat to Armstrong's chances for victory.
By Jerome Pugmire