AX-LES-THERMES, France -- Spain's Carlos Sastre captured Saturday's 13th stage of the Tour de France, but Germany's Jan Ullrich may be the bigger winner.

A weary Lance Armstrong clung to a diminishing lead in the Tour de France as a surging Ullrich powered away from him during Saturday's final punishing climb.

For the first time since Armstrong won the first of his four consecutive titles in 1999, the Tour was entering its final week too close to call.

Breakdown -- Stage 13

Stage Toulouse to AX-3 Domaines. A 122.5-mile race into the Pyrenees that featured a 9.4-mile climb over a pass called Port de Pailheres, towering at 6,603 feet above sea level. Winner Spain's Carlos Sastre in 5 hours, 16 minutes, 8 seconds. How Others Fared Germany's Jan Ullrich finished second in a powerful sprint and pulled closer to Lance Armstrong in the overall standings. Armstrong came in fourth, 10 seconds ahead of Alexandre Vinokourov in fifth place. American Tyler Hamilton finished 10th. Yellow Jersey Armstrong retains an overall lead of 15 seconds over Ullrich. Quote of the Day "One of the two will crack. There's going to be destruction in the days to come. It's going to be spectacular." -- French rider Richard Virenque, referring to Armstrong and Ullrich. Next Stage Sunday's 14th stage races deeper into the Pyrenees, with a 119-mile grind from Saint-Girons to Loudenvielle-Le Louron.

Ullrich, looking almost fresh while Armstrong appeared gaunt and exhausted, is second overall and only 15 seconds behind the Texan. The German had trailed by 34 seconds entering the 13th stage.

"This was a very difficult effort," said Armstrong, who was fourth in the stage.

Ullrich, the 1997 winner, was second in the stage and said his next goal is to seize the overall lead.

"I'm going to try to take the yellow jersey tomorrow," he said through a translator on French TV. "I'll see how I feel, and if all goes well, I'll do the maximum."

Spain's Carlos Sastre won Saturday's 122.5-mile race from Toulouse to the ski resort of Ax-3 Domaines. The ride featured a 9.4-mile climb over a pass called Port de Pailheres, towering at 6,603 feet above sea level.

Sastre, of the CSC team, stuck a baby's pacifier in his mouth as he finished in tribute to daughter Claudia, who turns 2 next month.

Ullrich was 61 seconds behind Sastre, who was timed in 5 hours, 16 minutes, 8 seconds. Spain's Haimar Zubeldia was third, 1:03 behind. Armstrong was next, 1:08 back.

Ullrich drew away from Armstrong in the last 5.6-mile climb to Ax-3 Domaines. His second-place finish in the stage resulted in a bonus of 12 seconds in the overall standings.

Despite his lackluster ride, Armstrong gained 10 seconds on Alexandre Vinokourov. The Kazak finished the stage in fifth place, falling 61 seconds behind the Texan in the overall standings.

"I'm not very disappointed," Armstrong said.

But he added that Ullrich "looks to be riding great, better and better every day."

"I'm just going to ride my rhythm and not let him get too far," Armstrong added.

Weakened by the persistent heat, Armstrong had difficulty recuperating from Friday's demanding time trial in which Ullrich prevailed. He had doubts about his ability to dominate Saturday.

"I didn't expect to have super legs -- yesterday was too hard. It was a really difficult effort," Armstrong said. "To recuperate ... is not possible in 24 hours, or 20 hours."

"At the start ... I thought, 'Uh-oh' it's going to be a bad day, but there are two more days left in the Pyrenees, and I still have a lot of chances," he said.

Ullrich is said to be battling a stomach problem, although he appears at the top of his game. He was 96 seconds ahead of second-placed Armstrong in the time trial, setting up a tense four days in the Pyrenees. Saturday was the first stage in the mountains separating Spain and France.

"One of the two will crack," French rider Richard Virenque predicted. "There's going to be destruction in the days to come. It's going to be spectacular."

Armstrong has been trying to assert himself in his attempt to equal Spanish rider Miguel Indurain's record of five successive Tour wins. His chance to stamp his authority on the three-week race could come Sunday when forecasts of rain in the Pyrenees may provide some cool.

Armstrong finds himself in the unusual position of having only a slender lead with seven days of racing to go.

At "other years at this point, no, I was lucky enough to have three or four or five minutes" advantage over competitors, he said.

"It's never been, obviously, this close," he added. "It's a different race, perhaps more exciting."

Virenque, who finished 13th Saturday and holds the polka dot jersey as the best climber, insisted Armstrong "is there and hasn't cracked yet."

But Armstrong knows he needs to build a bigger cushion over Ullrich in the Pyrenees before a final time trial July 26, the day before the Tour finishes in Paris.

Sastre was not ready to write off Armstrong.

"I don't think we can say he has weak points," he said. "He's really resistant. He's a big champion."

Information from the Associated Press was used in this article.