Vincenzo Nibali is refusing to take anything for granted despite seizing control of this year's Tour de France with his third stage victory on Friday.
The Team Astana rider stretched his lead at the top of the standings to three minutes 37 seconds after powering away from his rivals in the final phase of the hors categorie climb to Chamrousse.
Alejandro Valverde (Movistar) moved up to second place on what proved a dismal day for Richie Porte, with the Team Sky leader trailing in over nine minutes behind the leader and slumping to 16th place overall - having started the day second.
Nibali cemented his dominance in style as he gave chase to leading pair Rafal Majka (Tinkoff-Saxo) and Leopold Konig (Netapp-Endura) with 10km to go before bursting clear.
Majka claimed second place ahead of Konig, with Valverde 50 seconds down in fourth place.
Nibali, who also assumed the polka dot jersey from Joaquim Rodriguez (Katusha), said: "For the coming days, I only know that I have to remain quiet - there are still many mountains ahead.
"I've suffered a lot today because of the heat. At least as we were climbing, I felt better and better.
"My intention was just to control in the final climb but when I saw Richie Porte in trouble.. my goal became to gain important seconds on Valverde."
Italian Alessandro De Marchi (Cannondale) led an early breakaway but was hauled back into a peloton containing all the remaining race contenders on the final climb.
But Porte - promoted to Team Sky's principle rider following the retirement of Chris Froome - dramatically dropped off on a day which saw him fall out of contention 11 minutes 11 seconds behind Nibali overall.
Porte admitted: "I don't think I dealt with the heat very well. It's one of those things - it's a massive shame but we'll just have to see what happens tomorrow.
"I feel more for my team-mates who've been brilliant for me every day but if it happens to me I think it can happen to other guys too so I'll just have to keep on wishing."
Team Sky have dominated the last two Tours with wins for Sir Bradley Wiggins and Froome but team principal Sir Dave Brailsford conceded hopes of making it three in a row are over.
Brailsford told ITV: "We've had plenty of good days in the Tour de France over the last couple of years and today wasn't one of them.
"We've had two difficult days obviously with Chris crashing out, and everyone could see it was a tough day for Richie.
"We'll take stock tonight and see if we can get into a situation where we can animate the race - there's no point us just coming here and riding round.
"We've got to take the blows - it's not easy, it's disappointing - and think about the coming days in the mountains and what opportunities we've got to do other things in the race."
There is no respite ahead for Porte with Saturday's 177km stage from Grenoble to Risoul taking the riders over the Col d'Izoard - at 2,360m the highest point on this year's Tour.
And Valverde admitted he will need to recuperate if he is to start taking bites into Nibali's increasing lead.
He said: "I rode full gas to do the best I could but when Nibali attacked I couldn't follow him.
"Tonight I'm second overall. It's very well. Tomorrow will again be difficult. I will need to recover some strength.