After a string of setbacks earlier in the race, defending champion Alberto Contador struggled up the main climb and lost crucial seconds to other pre-race favorites for overall victory.
The 211-kilometer (131.1) trek from Cugnaux to the Luz-Ardiden ski station featured three tough climbs in the Pyrenees — including two that are among the hardest in pro cycling.
Sanchez and Belgian rider Jelle Vanendert overtook a group of breakaway riders in the final climb and held on, with the Spaniard winning their two-man sprint in the last several hundred meters (yards). Vanendert crossed 7 seconds later.
"It's incredible," Sanchez said of his first Tour stage win, after finishing fourth overall last year.
After leading a string of attacks on the other pre-race favorites, Frank Schleck of Luxembourg surged away and finished third — 10 seconds back — to vault into second place overall.
Italy's Ivan Basso was fourth, Cadel Evans of Australia was fifth, and Schleck's younger brother Andy was sixth, each 30 seconds behind Sanchez. Contador came eighth, 43 seconds back.
Voeckler gave the home crowd a delight for France's national Bastille Day holiday, clinging to the yellow jersey that he had expected to lose in the punishing climbs.
"I'm glad I was wrong," Voeckler said, with a smile. "It clearly wasn't expected. Keeping the jersey was far from expected as the stage started today." "You have to believe that the yellow jersey gives you a bit of added inspiration on the Bastille Day," he added.
Overall, Voeckler leads Frank Schleck by 1 minutes, 49 seconds, and Evans trails third, 2:06 back. Contador is seventh overall, 4 minutes behind.
"I was a bit careful," the three-time Tour champion said. "I saw the Schlecks were discussing together and that they were going to play their cards. Frank was the stronger — and both of them attacked." "But i'm nevertheless happy with this first mountain stage," Contador said. "Each day, I feel better ... I still don't have my best legs. I'm not riding with the same rhythm, but it's encouraging."
