"It's going to be one of the key days," two-time runner-up Cadel Evans said. "There'll be fireworks, don't worry."
The test comes today, a day after Norway's Thor Hushovd won the 13th stage and France's Thomas Voeckler kept the yellow jersey.
Today's stage is the third consecutive day in the Pyrenees. The 105-mile ride from Saint-Gaudens to Plateau de Beille starts with a sharp climb up Col de Portet d'Aspet followed by two ascents up Col de la Core and Col d'Agnes.
It finishes with a 10-mile climb to Plateau de Beille, a famed ascent that has a reputation for making, or breaking, contenders.
Although Evans and the Schlecks all took some time off three-time champion Alberto Contador in Thursday's 12th stage, the damage was not enough to seriously hurt Contador's chances.
"I will have to see my legs hold up and what the others do," the Spaniard said. "But if I get the chance, I will attack."
He said he had a "little niggle" in his knee at the beginning of the stage, but after that was fine. He banged the same knee in two crashes.
Contador finished Friday's stage nestled with Evans and the Schlecks. The contenders let others do the attacking on a 95-mile run that started from Pau and ended in Lourdes, home to one of the most famous Catholic shrines.
Voeckler leads Frank Schleck -- the older of the two Schlecks -- by nearly two minutes. But Voeckler is not a Tour contender and will probably lose the lead Saturday.
Frank Schleck is 17 seconds ahead of Evans, 28 seconds ahead of his younger brother Andy Schleck -- twice a Tour runner-up to Contador -- and 2:11 clear of Contador.