The Saxo Bank rider attacked twice in the finale to beat France's Laurent Lefevre and Italian Franco Pellizotti, who were second and third respectively 48 seconds behind.
Italy's Rinaldo Nocentini retained the overall leader's yellow jersey ahead of Spaniard Alberto Contador and Lance Armstrong, second and third respectively.
Sorensen featured in a seven-man breakaway at the 64-km mark that quickly built a gap of over four minutes -- and he proved the strongest of the group.
"I am extremely happy, it is something than I hardly thought of. I was lucky to win this one," said the 34-year-old Sorensen, one of Andy Schleck's domestiques on the Tour.
Andy Schleck and Cadel Evans also tried their luck early in the stage but they were swiftly reined in by the bunch.
