The 25-year-old Luxembourg rider says he knows he'll be on the spot in Thursday's "queen stage" of the race, needing to overcome his eight second deficit to defending champ Alberto Contador in the last day in the Pyrenees.
The 17th stage features three tough climbs along a 174km ride from Pau to an uphill finish on the Tourmalet pass - one of the most wrenching ascents in pro cycling.
Schleck knows three other stages loom before Sunday's finish in Paris. But for a climber of his pedigree, and with the Spaniard one the world's top riders in time-trials - one of which awaits Saturday - he has to make a move now if he wants to take home the yellow jersey.
"There's only one way and that is the climb of the Tourmalet," he said at a Saxo Bank team news conference on Wednesday's rest day.
This year's Tour marks the centennial of the race's first run in the Pyrenees, so race organizers gave it a particularly tricky challenge: two runs over the Tourmalet - over both sides.
"It's definitely the highlight of this Tour," said Schleck. "I always said the guy who has yellow tomorrow will have the yellow in Paris ... Tomorrow is definitely the queen stage."
Riders will also scale two category-1 climbs - the Col du Soulor and the Col de Marie-Blanque - the final three kilometers (two miles) of which are at a leg-straining gradient of more than 11 percent.
The ride starts at an altitude of 165 meters in Pau; by the finish, the racers will have scaled three successive peaks of about 1,000 meters, 1,500 meters and finally 2,100 meters.
With Schleck telegraphing his intentions to attack, Contador will know what's coming: The ultimate question on the day is whether the Spaniard will have the gusto and the legs to keep up.
One wild card will be the weather: driving rain showered the region of southwestern France overnight, and the forecast was for cool temperatures - which tend to dampen the appetite and ability to lead attacks.
Schleck says he'll need a lead of at least a one-minute margin over Contador coming out of the Pyrenees if he hopes to stop the Spaniard during the 52-kilometer time trial on Saturday.
Contador wasn't making assumptions about winning his third Tour just yet. He said Thursday's stage would be "very, very hard."
"We can have very big gaps in that stage, probably more than in the time trial."
For days, the race has been a two-man show, with pre-race hopefuls including Denis Menchov of Russia, Ivan Basso of Italy, and - most of all - seven-time champion Lance Armstrong, all seemingly to far back to threaten Schleck and Contador.
Spain's Samuel Sanchez is 2 minutes behind compatriot Contador, and Menchov is another 13 seconds back - gaps that could be overcome on such a big mountain stage.
Armstrong, who's ruled himself out of the running, could still mount a surprise: He's been gunning for a stage win in this his final Tour.
"It's not yet finished," said Johan Bruyneel, manager of Armstrong's RadioShack team. The Texan is in 25th place overall, 33:46 behind Contador. "We're going to try again."
Frenchman Anthony Charteau hopes to wrap up the title of King of the Mountains title - the polka-dot jersey. His closest chaser for that honor is 39-year-old countryman Christophe Moreau.
As for the yellow, despite bearing the strains of three tough days in the Pyrenees already, Schleck predicted he'll have a good ride on Thursday, and says he still has some energy left.
What gives me a lot of confidence is that I just feel I haven't given everything yet," he said. "I still haven't spoken my last word on this Tour."