The Astana rider won the stage by 10 seconds form Pole Rafal Majka of the Tinkoff-Saxo team

But Team Sky's Richard Porte, who was 2 min 47sec behind Nibali in the general classification going into the stage, struggled badly.

The Australian fell back with 12 kilometres of the stage remaining as the 18km climb took its toll.

He is now down in 16th in the overall standings, finishing almost nine minutes down on the winner, and is all but out of contention.

Richard Porte has slipped from second to 16th in the overall standings after badly struggling on the gruelling stage 13 ©AFP/Getty Images



With Spaniard Alejandro Valverde, who is now second in the general classification, finishing 50 seconds behind Nibali in fourth, the Italian has strengthened his grip on the yellow jersey.

He now holds a 3:37 advantage over the Movistar rider, with Frenchman Romain Bardet, who rides for AG2R La Mondiale, third overall, a further 47 seconds back.

Valverde had initially gone on the attack to win the stage, but Nibali instantly responded before launching an attack of his own to reel in the lead pair of Majka and Netapp-Endura's Leopold  Konig of the Czech Republic.

He assumed the lead with 3km to go and rode to victory.

Tomorrow sees the riders embark on a 177km stage from Grenoble to Risoul - stage 14 of 21.