Chris Froome has tasked his Sky team-mates with mission control as the Tour de France begins brutally tough back-to-back Alpine stages on Wednesday.

The reigning champion has an 18 second lead in the yellow jersey competition ahead of Italian Fabio Aru, but two other riders -- Romain Bardet and Rigoberto Uran -- are also within 30sec of the lead.

There are four categorised climbs on the menu on Wednesday's 183km 17th stage from La Mure to Serre-Chevalier in southeast France, and nearly 60km of steep climbing.

Although the finish is at the end of a long 28km descent, there are 30km of climbing just before that and Froome is determined to keep his rivals under tight wraps. 

"(Wednesday) is certainly going to be a race for us to control, but otherwise I'm feeling good," he said.

"I do believe these next two days are the biggest consecutive days in this year's Tour de France.

"It's hard to say exactly how selective it's going to be, or just a case of the four of us, all within half a minute, chasing each other's shadows, or whether it's going to be blown wide open.

"On the upside, myself and (team-mate) Mikel Landa are feeling great coming into this final week of the race."

Landa is fifth at 1min 17sec but could have an important role to play as a foil with Froome's rivals concentrating not just on trying to take time off the leader, but also cautious of Landa's potential to push them off the podium.

- Epic stage -

"Everyone is trying to get an advantage over Sky, which is a strong team which prefers to go on the attack rather than defend," said Basque Landa.

"Who knows, maybe (Wednesday) will require more attention, there are more climbs and more terrain for strategies -- I think it could be more dangerous."

It's certainly going to be an epic stage with three huge climbs -- the Col de la Croix de Fer, Col du Telegraphe and Col du Galibier, the highest point of this year's race at more than 2,600 metres above sea level.

It could encourage one of the top 10 rivals a bit further back to strike out a long way from home.

"I've lost time but we'll see how things go (on Wednesday)," said twice former winner Alberto Contador, who was caught out in Tuesday's crosswinds and lost more than 90sec, dropping out of the top 10.

"Everything will depend on how my legs are and how my rivals react during the race."

The Spaniard, though, is not one to sit on and wait to see how things pan out.

However, with a summit finish to come on Thursday and a time-trial on Saturday, Uran believes the leaders might be more concerned with avoiding losing time.

Uran is fourth at 29sec but knows that after Froome, he's the best time-trialist amongst the yellow jersey challengers.

If he losses no time in the Alps, he will fancy his chances of overhauling Bardet and Aru on Saturday.

"Now the Alps are beginning," he said. "You have to be careful every day."

Source: AFP