Tour de France leader Bradley Wiggins began a British-best seventh day in the yellow jersey on the 191-kilometre 14th stage from Limoux to Foix.
Wiggins will next Sunday aim to keep the maillot jaune on a permanent basis as the Tour's first British winner, but first must negotiate the Pyrenees and seven more days of racing.
The Briton held a two-minute-five-seconds-lead over Team Sky colleague and compatriot Chris Froome at the start of the day, with Vincenzo Nibali (Liquigas-Cannondale) 2mins 23secs adrift in third and defending champion Cadel Evans (BMC Racing) 3:19 behind.
Peter Sagan, Nibali's team-mate and the leader of the points classification, Steven Kruiswijk (Rabobank) and Sergio Paulinho (Saxo Bank-Tinkoff) attacked on the descent of the day's opening categorised climb, the category two Col du Portel.
The trio's lead was negligible, with Sagan's green jersey rivals keen not to allow him to accumulate more points, but the Slovakian's presence in the break suggested he might seek to help Nibali later in the stage.
Two category one climbs followed, with the concluding 3.5km of the second, the Mur de Peguere, on punishing gradient.
However, the summit of the Pyrenean peak was 38.5km from the finish, meaning any riders dropped would have an opportunity to respond on the run-in into Foix.
Sagan, Kruiswijk and Paulinho were joined at the front by eight others - Eduard Vorganov (Katusha), Sandy Casar (FDJ-Bigmat), Philippe Gilbert (BMC Racing), Cyril Gautier (Europcar), Gorka Izaguirre (Euskaltel-Euskadi), Luis-Leon Sanchez (Rabobank), Sebastien Minard (Ag2r La Mondiale) and Martin Velits (Omega Pharma-QuickStep).
The peloton were two minutes behind with 128km of the stage remaining.
The escape allowed Sagan to roll over the line first in an uncontested intermediate sprint as he added 20 points to his total. The peloton's deficit was more than 12:30 with 91km to go.