BARCELONA, Spain — A split second off the lead with the mighty Pyrenees looming, Lance Armstrong now gets to see what his rivals can throw at him.

After mostly flat, wind-swept stages along the Mediterranean rim this week, the Tour de France enters mountainous terrain that has long belonged to climbers.

Armstrong remained just a fraction behind overall leader Fabian Cancellara of Switzerland on Thursday after a treacherous, rain-soaked ride filled with crashes. Thor Hushovd of Norway led a mass sprint finish to capture the 113-mile sixth stage from Gerona to Barcelona.

The field faces a 139-mile haul from Barcelona to Andorra on Friday with a grueling uphill finish — the first and hardest of three days in the Pyrenees.

"Tomorrow is an important day," Armstrong said. "I don't know if it's the most important day, but it's definitely a big appointment on this Tour."

The seven-time champion said he was happy to emerge unscathed from Thursday's "nervous" ride. Two spills marred the last six miles — one involving Yukiya Arashiro of Japan, another involving former world champion Tom Boonen of Belgium, one of Hushovd's sprinting rivals.

Cancellara, a time trial specialist, acknowledges he's not the best climber and his six-day run in the front may soon end.

"What do I have to do tomorrow? It's a good question," he said. "It's been a beautiful week to be in this yellow jersey. ... I'm going to try to defend it but I don't know how well I can do."

Carlos Sastre, the 2008 Tour champion and one of the world's top climbers, clearly is on the radar screen of both Armstrong and Astana teammate and rival Alberto Contador. The 37-year-old Texan says the burden falls on riders like Sastre.

"I think the others will attack," Armstrong said. "We are in a position where we can wait and watch the others."

His Astana team is rich with talent, holding four of the top five spots behind Cancellara. Contador, the 2007 Tour champion and a world-class climber, is a close third, 19 seconds off the lead.

During his long championship reign on the Tour, Armstrong always made his mark by the first big mountains — methodically gaining on competitors while keeping an eye over his rear wheel.

Armstrong says the riders to watch are Andy Schleck, who is 1 minute, 41 seconds back; his brother and Saxo Bank teammate Frank Schleck, 2:17 behind; and Cervelo's Sastre, trailing by 2:44.

"I expect Carlos to make some accelerations," Armstrong said.

Such comments are surely part of a psychological game. The wild card is that Armstrong will also have to watch Contador, who was considered the favorite before the Tour began.

"I know Alberto wants to assert himself in the race. I don't need a team meeting to know that," Armstrong said.

"If he goes and nobody can hang with him, I'll just stay with the other leaders," he added. "But I'll show up tomorrow morning, try to do my best, get to the top as quick as I can, and we'll see."

The mountains figure to play a bigger role in this Tour. Friday's jaunt features the Serra Seca pass and a finish into Arcalis in Andorra that's so tough it's beyond classification in cycling's ranking system.

The granddaddy stage comes the day before the July 26 finale in Paris, with an uphill finish on Mont Ventoux, one of cycling's most legendary climbs.

So far in his comeback year after 3½ years in retirement, Armstrong has proved he can hold his own in time trials, like the one on opening day in Monaco, and gain time in the flats. The biggest question is whether he still has the legs to climb.

"I feel good, I feel strong, recovering well, relaxed," he said. "It's been a while since I've ridden at the front of a big mountain stage, but I'll be ready."

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AP Sports Writer Samuel Petrequin contributed to this report.