The 37-year-old Texan is second overall, but about to face his toughest test in this year's Tour as the grueling three-week race arrives in the Pyrenees mountains.
"That's my question mark, that's your question mark, that's everybody's question mark," he said. "But we don't have to wait long until we'll find out, that's the good thing."
Armstrong is again eliciting fear among his rivals after an astute move that earned valuable time over his rival and teammate Alberto Contador, plus an impressive show at the team time trial.
He couldn't be in a better position before heading into the mountains. But a lurch Friday in Arcalis, Andorra -- the finish of the first of three Pyrenean stages -- would bring him back down to earth.
Armstrong didn't take any risks during Wednesday's fifth stage with the mountains looming. He spent the day in front of the main peloton with his Astana teammates, trying to avoid any trouble on roads opened to strong winds.
To Contador's advantage, he's the best climber in the world and his legs are 11 years younger than Armstrong's. He's rejoicing that the race is finally reaching his usual playground.
"I can't wait to get to the mountains because it's my territory. The sensations are good, but I always like to confirm them," he said.
Because he's behind Armstrong, Contador probably won't get carte blanche from team manager Johan Bruyneel and won't be allowed to attack the Texan if he responds well in the climbs.
Armstrong, who underwent a doping test Wednesday, seems perfectly aware that Contador will be physically stronger in the mountains but believes his mental resources will prevail.
"Strength does not come from physical capacity. It comes from an indomitable will," Armstrong said on his Tweeter account, quoting Mahatma Gandhi.
A 122.1-mile trek from Le Cap d'Agde to Perpignan along the Mediterranean coast.
WINNER: French rider Thomas Voeckler, of the BBox Bouygues Telecom team, in 4 hours, 29 minutes, 35 seconds, in a solo finish.
YELLOW JERSEY: Fabian Cancellara, of Switzerland, who maintained his lead of a fraction of second over seven-time Tour champion Lance Armstrong.
QUOTE OF THE DAY: "I told Alberto that today was going to be complicated, so maybe he sees that I know what is going on in the Tour de France" -- Armstrong on teammate and potential title rival Alberto Contador, the 2007 Tour winner.
NEXT STAGE: Stage 6 takes riders into Spain for a 112.8-mile ride across plains along Gerona to Barcelona.