Tour de France: Vincenzo Nibali wins stage 13 to extend lead
Italian Vincenzo Nibali extended his overall lead in the Tour de France by winning stage 13 from Saint Etienne to Chamrousse.
The Astana rider proved strongest during the long final climb to come out on top ahead of Rafal Majka, who took second, and Leopold Konig in third.
Nibali holds an overall lead of three minutes, 37 seconds over Alejandro Valverde with Romain Bardet third.
Team Sky's Richie Porte, who began the day second overall, lost nine minutes.
Rob Hayles - BBC Sport commentator
"We expected Friday to be a day for the General Classification riders but we didn't expect them to lose so much time to Vincenzo Nibali. He is rubber-stamping his authority on this race.
"It was every man for himself on the final climb. With the time Richie Porte has lost today it is over for Team Sky, there's no way back from this. This is not to do with his form though. He was just having a bad day in the heat."
Porte, who took over as his team's leader following the withdrawal of reigning champion Chris Froome, fell back with 12km remaining of the 18.2km hors categorie ascent to Chamrousse, to drop to 16th in the overall standings, more than 11 minutes behind Nibali.
"I don't think I dealt with the heat," said Porte referring to the temperatures, which reached in excess of 35C.
"It's a massive shame but we'll see what happens on Saturday. My team-mates have been brilliant but if it happens to me it can happen to others, so I'll keep pushing."
The Australian was not the only rider to struggle in the searing heat in the Alps with Katusha's Joaquim Rodriguez, who was wearing the polka dot King of the Mountains jersey, also dropping away.
The Spaniard reacted by posting a number of crying emoticon faces on his twitter page.
The majority of the stage's excitement was crammed into the climb with Italian Cannondale rider Alessandro De Marchi, the last of a nine-strong breakaway to be caught around 13km from the summit.
Once Porte dropped away, the attacks began off the front of the bunch. Tinkoff-Saxo's Majka and Netapp-Endura's Konig both went clear of an elite group that contained race leader Nibali and Valverde.
Movistar's Valverde, who started the day third overall, then tried to ride clear of Nibali but the Italian countered the move and then sprinted off in pursuit of the leading duo.
"My intention was just to control in the final climb but when I saw Porte in trouble my goal became to gain important seconds on Valverde," said Nibali.
"For the coming days, I only know that I have to remain quiet - there are still many mountains ahead."
What to expect on stage 14
"It will be another hard day in the saddle and it could prove interesting because we go over 2,000m twice in one day and altitude affects different riders in different ways. The Col d'Izoard is the highest point in this year's race and the first to reach the summit will get the Souvenir Henri Desgrange, named after the race founder. We are right into the thick of the race now and there will be tired bodies. It's another day for the main contenders."
Nibali caught Majka and Konig with ease but rather than settling for a comfortable ride in their slipstreams to the finish, the Italian took off again with 3km remaining to further assert his authority on the race.
Valverde was unable to close the gap in the final kilometres and had to settle for fourth on the stage, but he lost a further 50 seconds on Nibali.
"I rode full gas to do the best I could but when Nibali attacked I couldn't follow him," conceded Valverde.
"Saturday will again be difficult. I will need to recover some strength.
Stage 14 is a second day in the Alps with three categorised climbs as the Tour goes over its highest point at the summit of the Col d'Izoard at 2,360m during a 177km race from Grenoble to Risoul.
Stage 13 result
1. Vincenzo Nibali (Ita/Astana) 5hrs 12mins 29secs
2. Rafal Majka (Pol/Tinkoff - Saxo) +10secs
3. Leopold Koenig (Cze/NetApp Endura) +11secs
4. Alejandro Valverde (Spa/Movistar) +50secs
5. Thibaut Pinot (Fra/FDJ) +53secs
6. Tejay van Garderen (US/BMC Racing) +1min 23secs
7. Romain Bardet (Fra/AG2R) Same time
8. Laurens ten Dam (Ned/Belkin) +1min 36secs
9. Jean-Christophe Peraud (Fra/AG2R) +2mins 09secs
10. Fraenk Schleck (Lux/Trek) Same time
General classification after stage 13
1. Vincenzo Nibali (Ita/Astana) 56hrs 44mins 03secs
2. Alejandro Valverde (Spa/Movistar) +3mins 37secs
3. Romain Bardet (Fra/AG2R) +4mins 24secs
4. Thibaut Pinot (Fra/FDJ) +4mins 40secs
5. Tejay van Garderen (US/BMC Racing) +5mins 19secs
6. Jean-Christophe Peraud (Fra/AG2R) +6mins 06secs
7. Bauke Mollema (Ned/Belkin) +6mins 17secs
8. Jurgen Van den Broeck (Bel/Lotto-Belisol) +6mins 27secs
9. Rui Costa (Por/Lampre) +8mins 35secs
10. Leopold Koenig (Cze/NetApp Endura) +8mins 36secs