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"A sprint is a question of centimeters ... you need just a little problem for it to go badly," he said. "But things are going positively."
Petacchi edged Estonia's Jaan Kirsipuu and Australia's Baden Cooke with an astounding burst of acceleration in the last 200m.
Armstrong cruised over the line with teammates George Hincapie and Pavel Padrnos.
Hincapie said the team's focus is on preparing Armstrong for today's opening mountain stage -- a 230.5km route from Lyon to Morzine-Avoriaz.
"Lance is one of the best climbers in the world," Hincapie said. "Our main strategy is to keep Lance out of trouble, and let him do the least work possible."
The Alps: Unforgiving conqueror of the frail, a challenge to relish for the likes of Armstrong.
Saturday's trek to the ski resort of Morzine-Avoriaz is the first of three days of alpine ascents. The riders face two mammoth climbs in the last 20km of the stage.
The Col de la Ramaz, which peaks at 1,619m, is followed by a 1,181m grind on the Cote des Gets.
Cooke, relaxing after his third-placed finish, explained what riders go through in a typical mountain climb.
"When the field is packed at the start of the race it's really hard. You go through a lot of pain trying to keep up," he said. "Once the pack splits, it's not quite so bad because it's a lot slower."
For a sprint specialist like Cooke, the mountains are even harder than for someone sleek like Armstrong.
"As a sprinter you're carrying an extra 5kg of muscle. It gets unbearable at times," he said. "You just have to put up with it."
Tomorrow's eighth stage, a 219km haul from Sallanches to L'Alpe d'Huez, is equally daunting.
What makes the mythical stage so notorious is that it finishes with a morale-shattering climb.
Riders will have already faced two huge challenges -- a 1,566m strain up the Col du Telegraphe, followed by a gritty 2,645m ascent up the Col du Galibier -- before entering the final ascent.
Cooke, riding in only his second Tour, has yet to tackle L'Alpe d'Huez -- it did not feature last year -- and is hardly looking forward to it.
"I only saw it on TV when I was younger," he said. "It may be a bit of a buzz going up it, but I'm more scared than excited to be honest."
Cooke says the best way to pull through is to talk about anything other than the mountain you're facing.
"I usually joke with my teammates that it's better than a nine-to-five job," he said. "I really know it's true. When I was injured one year, I did a few telesales jobs, I emptied parking meters ... that's worse than any mountain."
When all of France celebrates Bastille Day on Monday, riders will once again be suffering in the Alps.
Just for good measure, stage nine features two climbs exceeding 2,000m -- including the 2,360m Col d'Izoard.
In Thursday's stage, Armstrong's teammate -- Victor Hugo Pena of Colombia -- celebrated his 29th birthday by retaining the overall leader's yellow jersey.
Elsewhere, a rider for the ONCE team, Angel Vicioso, fell in the dash toward the finish.
He later completed the race, cradling his left arm and with a bloodied right knee.
Friday's sixth stage, from Nevers to Lyon, is a 230km trek ending in France's second-largest city.
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