Vincenzo Nibali tightened his grip on the 2014 Tour de France by riding away to his third stage victory on the 197.5km, 13th stage from Saint-Etienne to Chamrousse on Friday.

Nibali attacked 6.6km from home and put almost a minute into nearest rival Alejandro Valverde, who finished fourth on the stage but moved up to second overall.

Rafal Majka was second on the stage with Leopold Konig third while Australian Richie Porte, who started the stage second overall, cracked on the final climb to drop down the order.

Nibali proved he is simply too strong for the rest on a day when his Astana team looked vulnerable, with chief lieutenant Jakob Fuglsang crashing on a descent and Michele Scarponi struggling and dropping away on the penultimate climb.

But the Italian didn't need anyone, responding first to an attack by Valverde on the final uncategorised climb and then riding away from the Spaniard.

He caught Majka and Konig before leaving them in his wake in the final 3km to win by 10sec.

Valverde came in 50sec after the 29-year-old to drop to 3:37 behind the leader.

Young Frenchman Romain Bardet moved up to third at 4:24 after finishing the stage seventh, just 1:23 behind Nibali.

It allowed him to keep hold of the young rider's white jersey ahead of Thibaut Pinot, who is up to fourth at 4:40.

But the big loser was Porte who came over the line almost nine minutes after Nibali and dropped right out of the top 10, now more than 11 minutes off the pace.

STAGE 13 PREVIEW: Tour de France leader Vincenzo Nibali admitted he is wary of the apparent calm that has settled around his lead in the Grand Boucle.

Friday sees the peloton head into the Alps with a 197.5km 13th stage from Saint-Etienne to Chamrousse, including two brutal climbs at the end.

Nibali has been comfortably in control of the Tour since taking the leader's yellow jersey on the second stage with a daring solo escape.

He lost it last Sunday to Frenchman Tony Gallopin, but only for 24 hours before his victory on Monday's tough mountainous stage saw him don the jersey once again.

The Sicilian leads Australian Richie Porte by 2min 23sec with Alejandro Valverde of Spain third at 2:47.

But although he has not lost a single second to any of his overall rivals on any stage, Nibali says he cannot afford to take his eye off the ball ahead of the next two Alpine stages.

"There are still a lot of stages left to the end and we all know that every day there can be another surprise," he said.

"There are top class rivals such as Porte and Valverde who are the closest. 

Certainly, Nibali is more worried about that finish in Risoul than Friday's.

"I haven't done reconnaissance at these climbs. The one I fear more is the second stage because the first day you have more energy but the second requires more effort and takes more out of you.

"We will try to manage it with the team, we've had good days and we'll try to control things.

"It won't be easy for sure, people will attack me but I'll try to defend the yellow jersey and if I can take some seconds, I will try that."

Even so, the 13th stage cannot be underestimated as it ends with a first category ascent and then the uncategorised summit finish.

There are 138km before the 14.1km first category Col de Palaquit climb with an average gradient of 6.1 percent.

What makes that climb particularly difficult is the frequent changes in steepness ranging from 3 percent to 11.7 percent.

The hors category climb to Chamrousse is 18.2km long with a brutal 7.3 percent average but more consistent in its difficulty, although first half is slightly tougher.