VERBIER, Switzerland  Alberto Contador, the man who came into the Tour de France as Lance Armstrong’s biggest competition, could not sleep the night before the Tour headed into the Alps on Sunday.

Instead, he tossed. He turned. He thought about how he would measure up to the strength of other cyclists in Stage 15, a mountain stage earmarked by riders as a potentially decisive moment of this race.

As it turned out, though, Contador’s restlessness was needless.

In a stage that may have revealed the rider who will win this Tour, no one came close to Contador on Sunday. He won the leader’s yellow jersey and may wear it all the way to Paris, where the race ends July 26.

In the final three and a half miles of the 129-mile (207.5-kilometer) stage, Contador  who is known as the world’s best climber  made his move on the steepest ascent of the day.