Froome wary of Movistar pair Quintana and Valverde

Tour de France leader Chris Froome admitted he is concerned about Movistar pair Nairo Quintana and Alejandro Valverde, his closest challengers.

Colombian Quintana is second overall at 3min 10sec with Spaniard Valverde third at 4min 09sec.

But it was Movistar's tactics on Wednesday's 17th stage from Digne-les-Bains to Pra Loup that most impressed Froome.

While he was left alone without teammates on the final climb to the finish, Quintana and Valverde had two other Movistar riders for company.

"They rode a very good race tactically, also putting riders in the early breakaway today (Wednesday) so they could drop back once they'd crested the penultimate climb (and help their leaders)," said Froome, 30.

"It was a very impressive ride by them to have numbers in the finish there."

However, despite Froome being all alone, Tejay Van Garderen had quit the race through illness and Alberto Contador was two minutes back down the road after crashing.

Reigning champion Vincenzo Nibali was still in Froome's group but the Italian is more than eight minutes back in seventh place.

Froome said the only people he needed to worry about were Quintana and Valverde.

"For me the main concern was following Alejandro Valverde and Quintana -- those two were the only real two I needed to watch in that group," the Briton added.

"In essence, it made my job easier than if I had to think about eight or nine guys -- there were only two to focus on."

Froome, though, admitted he was surprised by the tactics not only shown by Movistar, but also Contador and Nibali.

The attacks started in the first 50km of the race before they increased as the stage progressed.

"It does give a taste of what to expect these next three days, there are three tough stages to go and my rivals are going to push me all the way to the finish, and they're also battling out amongst themselves for a podium finish."

Team Sky leader Froome said his rivals were running a risk by attacking so far out as they could blow a gasket and harm their own chances.

"I wouldn't say necessarily that it's desperation but I can see that my rivals are definitely going to take bigger risks," he added.

"I definitely feel as if attacking towards the beginning of the stage, that's a pretty big risk.

"If they do get away they're going to spend 100km up the road using energy.

"We're definitely seeing an all or nothing approach from the general classification contenders but they could pay for those efforts over the next few days."

But as the likes of Contador have no interest in finishing second, Froome is resigned to keep defending.

"Alberto Contador lost more time in that final descent with his crash and you can only expect him to attack in the next few stages."