VITTEL, France — Lance Armstrong is ready to climb again, ready to leave the pack at the Tour de France after days of flat riding that belonged to sprinters.

After three days of sitting back in the main pack while others challenged for stage wins, the worst thing to happen to Armstrong was a small puncture to his back tire in yesterday's 12th stage. Nicki Sorensen of Denmark won it, Rinaldo Nocentini of Italy kept the yellow jersey, and Armstrong's tire was repaired within a flash.

Finally, today, it's back to serious business as Armstrong goes up against his Astana teammate Alberto Contador on a tricky trek that features one grueling mountain climb.

"Tomorrow is hard, that is a real stage," Armstrong said yesterday. "The climb up Col du Platzerwasel is difficult, it is a long way. It is a longer day and anything can happen."

Armstrong, who retired after his seventh straight Tour win in 2005 only to stun the cycling world by announcing he would race again this year, expects some of the Tour contenders to make their move today.

12TH STAGE

A brief look at yesterday’s 12th stage of the Tour de France:
Stage: A 131.4-mile ride from Tonnerre to Vittel featuring six minor climbs.
Winner: Nicki Sorensen of Denmark, who clinched his first career Tour win at the age of 34. After a long breakaway, he surged ahead near the end and finished 48 seconds ahead of Laurent Lefevre of France. Franco Pellizotti of Italy took third place.
Yellow Jersey: Rinaldo Nocentini of the AG2R-La Mondiale team held on to the overall lead by finishing in the main pack with Lance Armstrong and Alberto Contador of Astana. Nocentini is the first Italian to wear the yellow jersey since Alberto Elli in 2000.
Quote of the Day: “I’m 34 years old now and it’s a big thing for me to perform at this level at this age. I started bike racing when I was 19 and I always hoped that I could maybe go on for many years and I think it shows today that it is possible.” — Nicki Sorensen.
Next stage: Today’s 13th stage is a 124.2-mile ride between Vittel and Colmar that features two difficult climbs: the Col de la Schlucht, and the Col du Platzerwasel.