The Agr-La Mondiale rider broke away from the leading group alongside Sandy Casar, Yaroslav Popovych and David Arroyo, who descended together from the highest peak in Tour history.
Dessel made his move 800 metres from the finish, and held his lead through a final right-hander 80m from the finish to take his first stage win in a Grand Tour, ahead of Casar, Popovych and Arroyo.
CSC were able to maintain control of the group containing Schleck who stays in the maillot jaune for another day.
The big loser was Rabobank’s Denis Menchov who struggled on the Bonette and drops one minute 13 seconds behind the Luxembourger in the general classification.
Schleck maintains his seven-second lead on Bernhard Kohl, with Cadel Evans a further one second back.
John-Lee Augustyn and Dessel shoot up the King of the Mountains classification, but Kohl stays in the polka dot jersey with a 25-point lead on Gerolsteiner team-mate Sebastian Lang.
Steffan Schumacher, another Gerolsteiner man, led over the first hors-categorie climb, the narrow Col de la Lombarde, and descended alone to Isola 2000, only to be caught 3.5km from the top of the Bonette by Augustyn and Dessel’s group.
Valjavec made the most gains overall, finishing 24 seconds back from his team-mate and cutting his deficit to five minutes 23 seconds.
Schleck remains in the box seat, 50 years since Luxembourg last produced a Tour winner, Charly Gaul. But with four riders — Kohl, Evans, Carlos Sastre and Menchov — within 73 seconds, his work is far from done.
Menchov and Evans are both stronger time-triallists, and with a stage against the clock from Cerilly to Saint Amand Montrond coming on the penultimate day, Schleck and Sastre’s CSC team must make more gains in the mountains. Their task begins today with the punishing climb over the Col du Galibier and steep summit finish at Alpe d’Huez.