A destructive and unplanned attack from the Texan sent him more than six minutes clear in the Tour standings, but he refused afterwards to accept that he had tied up the race.
With his second victory in three days, Armstrong finished 31 seconds clear of the man he feared, Alex Zulle of Switzerland, after breaking clear less than four miles from the finish in the heights of Sestriere.
"I always ride good in cold, wet conditions, which usually eliminate 50 per cent of the opposition who lose morale and have crashes," he said. "On the Galibier I was OK but I did not feel super, but the others looked like they were suffering.
"I am still nervous, and I don't think the race is over. The others can always come back, and I could have bad days. My form could run out in the next three or four days."
The 27-year-old so outclassed what was left of the bunch in the final climb that he evoked former greats such as Bernard Hinault, Miguel Indurain or his compatriot Greg LeMond, the only American to have won the Tour, in 1986, 1989 and 1990.
Yesterday's victory to remember was born out of defence: Armstrong did not want Spain's Fernando Escartin and the Giro d'Italia winner Ivan Gotti to gain too much time, so he chased them, caught them, then left them behind.
"I heard that I had a gap, and really I went too early in hindsight. Over the last two kilometres I was really suffering."
There was plenty of suffering behind him as the day's six mountains, including the mighty Col du Galibier, strung the field out along the rain- lashed 132 miles. The lone Briton, Chris Boardman, was among the bedraggled stragglers who clocked in more than 40 minutes after Armstrong.
Today, Armstrong's principal rival for the leader's yellow jersey is Abraham Olano, who struggled, finishing more than three minutes adrift. There was also a small triumph for Richard Virenque as he turned the screw tighter on the Tour organisation.
Because of the Frenchman's involvement in the doping scandal, Tour director Jean-Marie Leblanc had said that Virenque was not welcome on the Tour, but later had to rescind his decision under pressure from the sports governing body, the UCI.
He finally made it to the podium yesterday to pick up the red polka dot jersey of best climber, which he has won four times. Virenque, one of many shaken off by Armstrong, rode in sixth, and rose to ninth overall.
Among the non-finishers was the Italian Mario Cipollini, who ended the day with stitches in his head after a fall.
Super Mario, who last week became the first man for 60 years to win four consecutive stages, had arrived at the start yesterday in a golden chariot with his team-mates dressed as legionnaires. It was all publicity for their new golden racing strip, but Jacques Sabathier, the head of the race jury, saw red.
They had not presented the new jerseys and shorts for approval, and as their registered colours are red, Sabathier fined the team SFr6,000 (pounds 2,500). He also warned that if they arrived at the start today in similar style, they would not be allowed to start.
Considering that today's route ends with the tortuous, winding climb to the summit of l'Alpe d'Huez, that seems like an easy way out.
TOUR DE FRANCE RESULTS
NINTH STAGE (Le Grand Bornand to Sestriere, 213.5km, 131 miles): 1 L Armstrong (US) US Postal 5hr 57min 11sec; 2 A Zulle (Swit) Banesto +31sec; 3 F Escartin (Spain) Kelme +1:26; 4 I Gotti (It) Polti same time; 5 M Beltran (Sp) Banesto +2:27; 6 R Virenque (Fr) Polti; 7 C Contreras (Col) Kelme both s/t; 8 K van de Wouwer (Bel) Lotto +3:10; 9 A Olano (Sp) ONCE s/t; 10 L Dufaux (Swit) Saeco +3:30; 11 D Nardello (It) Mapei; 12 G Guerini (It) Telekom; 13 A Casero (Sp) Vitalicio Seguros all s/t; 14 B Salmon (Fr) Casino +3:43; 15 B Hamburger (Den) Cantina Tollo +3:46; 16 M Aerts (Bel) Lotto +4:24; 17 J Castelblanco (Col) Kelme +4:34; 18 S Garzelli (It) Mercatone Uno +4:51; 19 R Meier (Swit) Cofidis s/t; 20 C Moreau (Fr) Festina +5:04. Selected: 23 P Tonkov (Rus) Mapei +5:04; 60 M Boogerd (Neth) Rabobank +18:02; 137 C Boardman (GB) Credit Agricole +40:29.
Leading overall standings: 1 Armstrong 39hr 31min 7sec; 2 Olano +6min 3sec; 3 Moreau +7:44; 4 Zulle +7:47; 5 Dufaux +8:07; 6 Nardello +8:36; 7 Casero +8:51; 8 Escartin +9:01; 9 Virenque +10:02; 10 Tonkov +10:34; 11 Salmon +10:42; 12 A Peron (It) ONCE +11:13; 13 Hamburger +11:30; 14 Guerini +11:39; 15 Garzelli +12:10; 16 Aerts +13:20; 17 K Livingston (US) US Postal +15:41; 18 Van de Wouwer +16:14; 19 T Hamilton (US) US Postal +16:52; 20 A Galdeano (Sp) Vitalicio Seguros +17:29. Selected: 57 Boogerd +34:23.Reuse content