Chris Froome surrendered the yellow jersey to Fabio Aru as Romain Bardet won stage 12 of the Tour de France in Peyragudes.
Froome faded badly on the brutal final climb of the 214.5km stage from Pau while Aru finished third to take the final bonus seconds on offer.
The Italian national champion now leads the Tour by six seconds from Froome, with Bardet closing to within 25 seconds of yellow in third place overall.
Ireland's Dan Martin finished ahead of Froome in sixth on the stage, moving up to fifth overall, 1:41 behind Aru.
A tough day in the mountains finished with a climb of the airstrip at Peyragudes, only a few hundred metres long but with gradients nearing 20 per cent.
The group of favourites came to the foot of the climb together, but after Aru struck out first, Froome struggled to respond.
Frenchman Bardet timed his attack to perfection, taking victory ahead of Aru and Colombian Rigoberto Uran, both two seconds back but picking up bonus seconds.
"It's a great joy. I had good legs but I was patient," said AG2R-La Mondiale rider Bardet.
"The team has never been so strong, we are a real factor in the race. Now we have to do our best in the final part."
Top 10 GC after stage 12 #TDF2017🇫🇷 pic.twitter.com/UvsB8AZB4K
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Froome finished a lowly seventh, 22 seconds behind Bardet. Team Sky had held the yellow jersey since Geraint Thomas' time trial win in Dusseldorf on the opening day.
"I didn't have the legs. It's a nice victory for Romain Bardet, and hats off to Fabio Aru for taking the yellow jersey," said Froome, whose team had contained the opposition until the final effort.
"I did my best but I didn't have the legs to follow."
Froome began the day with an 18-second lead over Aru and 51-second advantage over Bardet, but the picture looks radically different ahead of Friday's short stage from Saint-Girons to Foix, which has the potential to change the shape of the race once again.
The fireworks in the fight for yellow came after Stephen Cummings fell short in a bid for a solo win on the 50th anniversary of Tom Simpson's death during the 1967 Tour.
Cummings had been part of a 12-man break which went away early on the stage, and he attacked solo on the climb of the Port de Bales, staying clear until midway up the Col de Peyresourde, caught with eight kilometres of the stage remaining.
