Lance Armstrong, Alberto Contador on their guard as Tour de France heads back into mountains

VITTEL, France — After three stressful flat stages marred by crashes, the Tour de France is heading back into the mountains.

Friday’s treacherous ride features two difficult climbs as an appetizer for what’s shaping up to be a crucial Alpine stage over the weekend — where the rivalry between Astana teammates Lance Armstrong and Alberto Contador will most likely re-ignite.

After fighting hard in the Pyrenees, the seven-time champion and his younger challenger put their quarrels aside this week in the plains, leaving the headlines to British sprinter Mark Cavendish — who clinched two consecutive stage wins — and Nicki Sorensen of Denmark, winner of Thursday’s 12th stage.

Despite a tire puncture on his way to Vittel, Armstrong remained in third place overall, 8 seconds behind race leader Rinaldo Nocentini of Italy. Contador, winner of the 2007 Tour, is in second place, 2 seconds ahead of Armstrong.

Armstrong and Contador lost a precious teammate, American rider Levi Leipheimer, who fell off his bike about 1.8 miles from the line in a crash involving two-time Tour runner-up Cadel Evans. The American withdrew from the race, the Astana team said Friday morning.

“Woke up to bad news. Levi is out with a broken wrist,” Armstrong said on his Twitter feed.

After puncturing his back tire, Armstrong rapidly caught up to the peloton with the help of his teammates and finished the stage in the main pack with Contador and Nocentini.

“Up and down all day long and was aggressive from the start,” Armstrong said on his Twitter feed. “Took 80 (kilometers) for the break to go. Ouch. Also pretty warm.”

The 37-year-old Texan is likely to endure more pain on Friday’s 13th stage, a 124.2-mile ride between Vittel and Colmar, featuring two big climbs — the Col de la Schlucht and the Col du Platzerwasel.

Those mountain passes are probably too far away from the finish line to be decisive in the battle for the yellow jersey, but Armstrong promises to be on his guard. The Texan has bad memories of the Col de la Schlucht, where he found himself completely isolated from his team and under attack from rivals during his last winning Tour in 2005.

“Tomorrow is hard, that is a real stage,” Armstrong said Thursday at his team bus. “The climb up Col du Platzerwasel is difficult, it is a long way. It is a longer day and anything can happen.”

That stage could have been even more interesting if Tour organizers and the International cycling federation hadn’t decided to overturn an earpiece ban scheduled for Friday under pressure from riders and team directors.

“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.”

Andy Schleck is within two minutes of Nocentini’s lead, 2008 winner Carlos Sastre trails by 2 minutes, 52 seconds, and two-time runner-up Cadel Evans is 3:07 off the pace.

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

Asked whether he will attack on Friday, the 26-year-old Contador refused to unveil anything after being accused of going against his team’s interests when he leapfrogged Armstrong overall following a bold move in the first Pyrenean stage last week.

“We’ll have to see how the race will evolve, because it’s up to the others to attack,” Contador said. “I’m in a position where I can wait and see.”

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

Sorensen was part of a group of seven riders that finished several minutes ahead of the main pack after riding 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.”

AP Sports Writer Jerome Pugmire contributed to this report.