Britain's David Millar claimed his fourth Tour de France stage, and first since 2003, after dominating a two-up sprint with Frenchman Jean-Christophe Peraud on Friday.
Yellow jersey holder Bradley Wiggins finished nearly eight minutes behind a leading group of five to retain his race lead after the 226 km 12th stage between Saint-Jean-De-Maurienne and Annonay in the Ardeche.
Wiggins' Sky teammate Chris Froome stayed second overall at 2min 05sec with Vincenzo Nibali (Liquigas) in third at 2:23 and Australia's Cadel Evans (BMC) fourth at 3:19.
Millar's fourth career stage on the race comes in the wake of an “horrific” Tour campaign for his Garmin-Sharp team who have lost several riders including team leader Ryder Hesjedal.
“It's massive. It's been a horrific Tour for us so far,” said Millar.
“I really wanted to do something... prove that we're still here and show that Garmin-Sharp are one of the best cycling teams in the world.”
A day after the toughest stage in the Alps and with the only two major climbs coming within the first 80 km, the breakaway contenders were primed from the start of the race's longest stage.
After 20 km of attacks and counter-attacks a 19-strong group that included the first five finishers managed to pull itself free of the peloton to begin the 12.5 km climb to the summit of the Cucheron.
On the way to the summit the frontrunners split as the demands of the category one climb took their toll.
Eleven leaders went over the summit with a lead of 53sec on the main peloton containing the yellow jersey and the big favourites and they would go on to increase their lead further.
Millar had been part of a five-man group that finally broke free of their companions after the descent of the day's second climb and with around 120 km to race.
By then, Millar was already considering his options.
“When it whittled down to the five riders I didn't expect to be at the front but I was feeling great,” he said.
“Once we were in the group of five I knew I was the fastest sprinter there so I decided my tactics about 120 km out, and that was to win the sprint.”
They went on to build a significant lead on the peloton being controlled by Wiggins' Sky team, taking their advantage to nearly 13 minutes.
Although there was little at stake for the yellow jersey challengers on this stage, Millar denied the peloton had sat up to let them race away.
“They didn't let us go, we really had to fight hard to build that advantage!” added the Scot.
After collaborating all day, Spaniard Egoi Martinez of the crash-hit Euskaltel team began the hostilities with a brief attack 4 km from the finish.
But it was Peraud's acceleration 2.7 km from the slightly uphill finish that proved decisive.
Millar fought hard to get back on the wheel, and his efforts allowed the pair to open up a decisive gap on Martinez, Frenchman Cyril Gautier and Croatian Robert Kiserlovski.
At the end, Millar collapsed on the ground to soak up a victory that comes 45 years, to the day, after the death of former British cycling giant Tom Simpson.