Cadel Evans still backing himself to challenge Chris Froome in the 2013 Tour de France

CADEL Evans believes Sky's Tour de France stranglehold is under threat unless race leader Chris Froome delivers a killer blow in the time trial.

CADEL Evans believes Sky's supposed Tour de France stranglehold is under threat unless race leader Chris Froome delivers an expected killer blow in the stage eleven time trial.

Froome is an outstanding time trialler and is expected to dominate the 33km race of truth from Avranches to Mont-Saint-Michel.

But both 2011 winner Evans and dual champion Alberto Contador are desperate for time gains as the centenary tour  heads to the North Atlantic coast.

Evans, 4 min 36 sec behind Froome, has had some of the best results of a stellar career in time trials and is predicting more change at the top after Sunday's bruising stage in the Pyrenees.

"Movistar and Saxo Bank had the strongest teams," Evans said in his diary. "Which could change the direction of the race, or at least people's thinking of the direction of the race.

"We'll see when we get to the TT and the Alps."

Sky's invincibility was destroyed on stage 9 as Richie Porte, Geraint Thomas and Peter Kennaugh suffered and could not protect Froome.

Froome coped superbly in the crisis to retain the yellow jersey and a 1 min 25 sec lead over Movistar's Alejandro Valverde.

Evans finished sixth on the stage after making a brilliant recovery from his dismal performance on stage 8.

The Victorian revealed he had two bike changes after a rival rider accidentally bent his rear derailleur just as Movistar launched its attacks.

Down 1 min 51 sec, Contador said he would continue to chip away at Froome's lead - probably next week when a punishing climb to Mont Ventoux is followed by three days of serious climbing in the Alps.

"Of course, I am going to try something," Contador said. "It's a very flat time trial and that is a disadvantage for me. We need to know when to take risks, especially when everyone is struggling.

"Throughout my career I've found my best form in the third week."

Drugs dominated discussions on the rest day when both Froome and Contador were questioned on the subject.

Contador said he had never doped, despite being stripped of the 2010 Tour title after testing positive for banned clenbuterol.

"You can believe what you want," he said. "But the only thing that I can tell you is that I have always practiced cycling clean."

Refusing to be drawn on Contador, Froome said: "There definitely do need to be questions asked about performances in the past, where guys have been very successful and are no longer on that same level."