CASTELSARRASIN, France - The reputation of the Tour de France will rest on the shoulders of 24-year-old Alberto Contador when the 18th stage starts from Cahors on Friday.

Alberto Contador of Spain is being kissed on the podium after the 17th stage of the 94th Tour de France cycling race between Pau and Castelsarrasin, Thursday, July 26, 2007. Contador took the overall lead. [AP]

The Spaniard took the yellow jersey following the 17th stage, after the exclusion of previous race leader Michael Rasmussen, who was sacked by his Rabobank team for lying about his whereabouts during training, an allegation the Dane denies.

Rabobank decided to continue in the Tour but lost Denis Menchov on Thursday when the Russian pulled out halfway through the stage.

The credibility of the race has been further tested by positive dope tests for Kazakh Alexander Vinokourov and Italy's Cristian Moreni.

"I have never tested positive otherwise I would not be there", Contador told reporters.

"I have never missed a test, in or out of competition. I am clean."

The Discovery Channel rider should retain his yellow jersey when the stage ends in Angouleme after a relatively flat 211-km ride in southwestern France.