PATRICK HERTZOG / AFP/Getty Images

Seven-time Tour de France winner Lance Armstrong of Team Astana prepares to begin the 12th stage Thursday.

updated 2:41 p.m. ET July 16, 2009

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 on Thursday’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, on Friday, 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 Thursday. “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 on Friday.

“You have to watch all the rivals, even someone like (Denis) Menchov,” Armstrong said of the Giro d’Italia winner. “Some might say he is five or six minutes behind and his race is finished, but if he gains back time, he has the Alps, and then if he is close enough on the (Mont) Ventoux, he could present a problem.”

Armstrong briefly looked to be in trouble after about 37 miles on Thursday, when he had to pull over to let his Astana team repair a puncture in his back wheel.

But after a few moments, four of Armstrong’s teammates helped him catch up with the main pack again.

“Up and down all day long and was aggressive from the start,” Armstrong said on his Twitter feed.

Although Nocentini will keep the yellow jersey heading into Friday’s 13th stage, he is not considered a threat for overall victory — and seemed to be saying he’s done the best he can.

“It’s a tough stage tomorrow but I’m already really happy,” Nocentini said.

He leads Contador by only six seconds and Armstrong by eight.

“We are approaching the really hard stages,” Contador said. “It will be a hard day (Friday). Then we will see how things develop with the uphill finish (to Verbier) on Sunday. In theory I should be OK, because it’s up to the others to attack.”

Those ’others’ are merely the 2008 Tour winner Carlos Sastre, two-time Tour runner-up Cadel Evans, and Andy Schleck — all of whom are lurking behind Contador and Armstrong.

Schleck is within two minutes of Nocentini’s lead, Sastre trails by 2 minutes, 52 seconds, and Evans is 3:07 behind Nocentini.

They could well choose to launch an attack on Friday’s 124.2-mile ride between Vittel and Colmar, which also features a tough climb up the Col de la Schlucht.

“I would put Carlos, the Schleck brothers and Evans in the most dangerous category,” Armstrong said.

On Thursday, Sorensen earned the first stage win of his Tour career by breaking away and finishing well ahead of Laurent Lefevre.

Sorensen was part of a small group of seven riders that finished several minutes ahead of the main pack after foraging ahead unchallenged during the 131.4-mile trek from Tonnerre to Vittel, which featured six small hills.

The Danish veteran, after years as a support rider in Bjarne Riis’ team, finished 48 seconds ahead of Lefevre.

“I’m 34 years old now and it’s a big thing for me to perform at this level at this age,” Sorensen said. “I started bike racing when I was 19, and I always hoped that I could maybe go on for many years.”

The chasing pack, including Nocentini, Armstrong and Contador, finished nearly six minutes behind him.

“We let the breakaway go after four minutes,” Nocentini said.

Astana rider Levi Leipheimer fell off his bike about 1.86 miles from the line in a crash involving two-time Tour runner-up Cadel Evans. Leipheimer had cuts and bruises on his right wrist, shoulder and back. He hopes to resume on Friday.

“My wrist hurts, but surprisingly it’s OK. It could have been a lot worse,” Leipheimer said. “I was a bit surprised by a left corner ... my tire was sliding and I couldn’t quite save my bike from sliding out.”

Also Thursday, the International Cycling Union said in a statement that an earpiece ban set for Friday’s 13th stage from Vittel to Colmar has been overturned, and riders will be able to race with electronic radio equipment as normal.