Twelve days in to the Tour, and the race today finally hits the mountains. Who scales the heights the fastest will give us the first indications of which cyclist will be standing tall in Paris in just over a week's time.
Riders traditionally fear the first mountain stage more than any other and not just because today's three Pyrenean monster climbs, culminating with a 13-kilometre ascent to Luz Ardiden, will be an exceptionally brutal test.
For the favourites, the key issue will be how they react to the radical change of pace to 4,000 metres of climbing in a day (just think of pedalling up Ben Nevis three times in six or seven hours and you get the picture). It's a change so dramatic it means that any chinks in their armour that have developed over the previous 11 stages risk turning into yawning gaps.
However, while the favourites – now reduced, after the spate of accidents, to Alberto Contador, Cadel Evans and the Schleck brothers – are so experienced they can anticipate their own weak points, for those making their first challenge, the first major mountain climbs will feel steeper than ever.
"It's the day that you see in which group you really belong," says the 26-year-old Slovak Peter Velits, who is seventh on general classification.
Since finishing 32nd in his only previous Tour back in 2009, Velits – whose twin Martin also races for HTC-Highroad – has risen to outsider status after finishing third in the 2010 Tour of Spain. "You have to be realistic, you can't say: 'I'll stick with Contador and get second in Paris.' But I'm healthy, I'm happy with where I am overall, I haven't crashed and although I'm not fresh anymore, who is fresh after 10 days like we've had in this year's Tour?"
For Velits, the switch from flat to mountain has never been a problem physically – "It's more something for my head than my legs" – the HTC-Highroad team doctor, Helge Riepehof, says that in yesterday's stage the Slovak was "testing himself with extra efforts. In a sense, for the top guys these first 11 stages are like hard training, but with efforts of perhaps 40 seconds max. By going in really deep yesterday they were showing their bodies it's time to work in the mountains."
"You can only rate your form when you've done that first big climb," Velits says. "It's only then that I'll know if I've got the same kind of condition as I had in last year's Tour of Spain. But I don't lose sleep over it. I've done the work, we've known this is coming."
Although Luz Ardiden is unlikely to determine this year's Tour, it has been the defining climb in other years. In 1990, it was where Greg LeMond set himself up for his third and final victory, although the man who succeeded him on the Paris podium, Miguel Indurain, actually took the stage win.
In 2003, Luz Ardiden confirmed that Lance Armstrong – despite numerous setbacks, including being whacked by a spectator's bag at the foot of the climb – was going to beat Jan Ullrich in what proved to be the Texan's toughest Tour.
Although the French will be hoping for a local winner to aid their Bastille Day celebrations, as defending champion, Alberto Contador remains the benchmark by which all others will test themselves. However, his form has been under scrutiny after an exhausting Tour of Italy, as well as three crashes and a knee injury in this Tour.
"If you asked me to predict a podium in Paris, I'd say: Contador, Evans and Andy Schleck," Velits says. "But in which order, I don't know, nobody knows: we're only half way. Personally, I'd like to just be as high up the standings as possible." And for him, like everybody else, the first real indication of his chances will come today.