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Chris Froome continued his march towards a maiden Tour de France title when he beat fierce rival Alberto Contador by nine seconds to win the 17th stage, a 32km time trial.
Team Sky rider Froome, who has now three stage wins to his name in the Tour this year, clocked 51 minutes 33 seconds to extend his overall lead over second-placed Contador to 4:34 going into three gruelling stages in the Alps.
"It's a surprise for me to win today," Briton Froome told reporters.
"The first descent was dangerous and technical, I did not want to take risks.
"I was a bit down in the first two time checks but I feel the bike change helped me towards the end," added Froome, who switched to a time trial bike just before the second time check.
Spain's Joaquim Rodriguez took third on the day one second behind Contador as Dutchman Bauke Mollema lost his third place overall to Czech Roman Kreuziger, who trails Froome by 4:51 after finishing fourth.
The time trial featured two second-category climbs and two descents, the first one being particularly tricky.
Contador went full throttle after opening a two-second gap on the first time check but Froome, who on Tuesday accused the Spaniard of careless riding, had more in the tank.
"I'm very happy to see that both Roman Kreuziger and Alberto Contador did such a stunning time trial and we have now conquered the two lower spots of the podium," Saxo-Tinkoff sports director Fabrizio Guidi told reporters.
"Tomorrow and the next days we will continue our plan and follow our objective, which is to win the Tour de France."
France's Jean-Christophe Peraud, ninth overall before the start of the stage, had a nightmarish day. He broke his right collarbone in a crash during a training ride but decided to take the start, only to come off his bike again two kilometres from the finish.
Unsurprisingly, Peraud pulled out of the race.
Thursday's 18th stage takes the peloton from Gap to l'Alpe d'Huez, the 21 hairpins to the resort being climbed twice with the treacherous descent from the Col de Sarenne sandwiched in between.
Up to one million spectators are expected on the ascent, according to the local office of tourism quoted by French TV.
With rain forecast, there has been media speculation the stage could end after the first ascent of l'Alpe d'Huez to avoid the descent from the Col de Sarenne but organisers have declined to comment.
Although he would be disappointed should the second climb up l'Alpe d'Huez be cancelled, Froome believes safety has to come first.
"That's a dangerous descent as it is so if it starts raining I will hope that they cancel it.
"I do think it's a good idea, it's a dangerous descent," he said.