The peloton will tackle the punishing ascents of the Croix de Fer, Col du Telegraphe and Col du Galibier before a downhill finish in Serre-Chevalier in Wednesday's 17th stage.

Le col d'Izoard, un des lieux mythiques des Alpes.

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But the 18th stage will be the crucial one, ending up the lung-busting Col d'Izoard, where the "first big battle of the climbers" will be staged according to LeMond, a Tour winner in 1986, 1989 and 1990:
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It will come down to Izoard, it will be the first big battle for climbers to win the Tour. We've never seen a really all-out day of racing since the start. In Peyragudes, they just raced in the last 500 metres.
Wednesday's downhill finish, with a possible headwind, would make it hard for a lone rider to go all the way to the line, according to LeMond.
It's not as explosive as you think (on Wednesday). The Croix de Fer is really hard and the Galibier is really long, but it will be hard to put time into Froome.
Chris Froome leads Italian Fabio Aru by 18 seconds and France's Romain Bardet by 23, with Colombian Rigoberto Uran in fourth place, 29 seconds off the pace.
The Briton has a virtual extra advantage as he is expected to beat his rivals in the final time trial on Saturday.
"It seems everybody will wait until the last day with the mountain-top finish," said LeMond. "I think they will race conservatively tomorrow and wait for the Izoard."

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Last year's runner-up Bardet might hold the key as he has been the most aggressive rider this year, while LeMond believes Uran is also in an ideal position
I think Bardet will go big. Uran is going to surprise, he's been riding well and he has no pressure. Bardet, on the other hand, is under pressure. There are a lot of expectations.
No Frenchman has won the Tour since Bernard Hinault claimed the last of his five titles in 1985.
The dark horse will be Spain's Mikel Landa, who is fifth overall 1:17 behind his Team Sky leader Froome. He has been instructed to work for the Briton but has sometimes looked stronger than him.
Landa's contract with Sky expires at the end of the season and he is expected to sign for the Spanish team Movistar.
"Landa has to ask himself if he wants to give the Tour de France to someone who will be his opponent next year," LeMond concluded.