LYON, France -- Monsters for some, manna for others, the mountains of the Tour de France offer Lance Armstrong a chance to power away from rivals as he goes for a fifth straight title.

Armstrong was second overall, behind a teammate, after Friday's 142.83-mile sixth stage from Nevers to Lyon.

	Victor Hugo Pena, wearing the yellow jersey, retained the overall lead Friday.

But except for a couple of noteworthy hills, the stage -- like the five others over the past week -- was relatively flat. The Alps, where the riders head Saturday, are entirely different.

"Until now it's been a psychological game to see where everyone is situated,'' said Johan Bruyneel, sporting director of Armstrong's U.S. Postal Service team.

From Saturday, Bruyneel said, "the real Tour de France begins.''

Among those expected to struggle in the climbs is Italian rider Alessandro Petacchi, the sprinting star of the centennial Tour's first week. Finding hidden reserves of strength, he swept past opponents in the dash at the finish Friday, taking his fourth victory in six days of cycling's premier event.

Armstrong finished 39th, saving energy for the mountains, where he excels. His U.S. Postal Service teammate Victor Hugo Pena kept the top spot in the overall standings -- thanks to a one-second lead over Armstrong.

But Pena, the first Colombian to wear the overall leader's yellow jersey, said that in the Alps, the Postal team's goal is to help Armstrong equal the record of five successive victories held by Miguel Indurain.

"I'm going to do my job as a teammate to make sure Lance wins his fifth Tour de France,'' Pena said. "Lance Armstrong will have the yellow jersey.''

The 143.14-mile haul Saturday from Lyon to the ski resort of Morzine-Avoriaz is the Tour's longest stage and the first of three days of alpine ascents. Four big climbs come later in the Pyrenees.

Saturday's route includes the 5,342-foot Col de la Ramaz, not the Tour's biggest climb but daunting nonetheless.

"A lot of people will look at it as a medium mountain day, not a monumental day,'' Armstrong said. "But just moving on a mountain is very hard.''

The Ramaz ascent is "the first real climb,'' Bruyneel said. "It's a chance to see who is in good shape and who's not.''

Bad days in the mountains can shape a Tour's outcome.

Armstrong says he was lucky not to lose the event in 2000 on a climb to Morzine, when he ran out of energy because he hadn't eaten enough. He struggled on, losing more than a minute to key rival Jan Ullrich, the 1997 Tour winner putting up a strong challenge again this year. "It was the worst day of my life on a bicycle,'' the 31-year-old Texan said in his autobiography.

Petacchi, awesome on flat finishes such as Friday's, was among the riders not relishing the prospect.

"As soon as the road begins to climb, it's very difficult for me,'' the Italian said.

Petacchi's victory secured him the green jersey for the fastest sprinter. He said that if he gets over the Alps, he hopes to still be wearing it at the finish in Paris on July 27.

"I know it's something very special, what I'm doing here,'' said Petacchi, who also won sprint finishes in stages 1, 3 and 5. "It's really historic.''

He passed Baden Cooke of Australia and Fabrizio Guidi of Italy at the line.

Petacchi, who rides for the Fassa Bortolo team, completed the trek from Nevers in 5 hours, 8 minutes, 35 seconds, averaging 27.7 mph despite 90-degree heat.

The stage, the Tour's second-longest, included a trying ascent of the 2,336-foot Cote des Echarmeaux, about 45 miles from the finish.

Petacchi said he struggled, but his teammates helped shield him from the wind so he could stay with the pack.

"My companions have again done an extraordinary job,'' he said. "They brought me into a front position in the descent from the hills for the sprint -- and I thank them.''

"I don't know where I got the energy because I was going flat-out,'' he said. "But when I see the finish line my strength comes back.''