Alberto Contador admits he expects to come under attack at every possible turn as he bids to defend his yellow jersey at the 97th edition of the Tour de France.

Contador, the champion in 2007 and 2009 who also won the Tours of Italy and Spain in 2008, will saddle up for the start of the three-week epic today as the favourite to top the podium in Paris on July 25.

However the Spanish climbing specialist knows that obstacles over which he has little control - attacks from rivals, crashes and even the weight of living up to expectations - will make his task doubly harder.

"Nothing is more difficult than winning a race for which everyone says you're the favourite," he said.

"You know that any moment of weakness will be exploited by your opponents."

With a total of six high mountain stages, one long time trial and no team time trial, the race looks tailor-made for Contador.

The Astana rider is ranked just ahead of 2009 runner-up Andy Schleck, with Lance Armstrong among a handful of challengers that also includes Italian Ivan Basso, Britain's Bradley Wiggins and Australian Cadel Evans.

However, Contador's outlook could all change as of Tuesday, depending on what damage has been done by the third stage's 213km ride from Wanze in Belgium to Arenberg in France which features seven sections of cobblestones.

Contador inspected the stage a few months ago in the company of retired Belgian one-day specialist Peter Van Petegem.

But some rivals believe it was a futile exercise that will mean nothing when the jostling to get to the front ahead of the cobblestones will prompt panic, cause crashes and leave many top contenders counting their losses.

"Someone will lose the race that day, I just don't know who it's going to be," said Garmin-Transitions team boss Jonathan Vaughters.

"It could be Lance (Armstrong), it could be Contador, could be (Andy) Schleck. Whatever happens, there will be some happy people that day, and some not so happy."