Defending Tour de France champion Chris Froome declared "the race is certainly on now" after he lost the yellow jersey to Fabio Aru on stage 12.
Froome went into the 214.5-kilometre ride from Pau to Peyragudes with an 18-second advantage over Astana's Aru.
It looked as it Team Sky had engineered a perfectly planned race for Froome on an exhausting six-hour route that featured two category-one climbs - either side of the hors-category ascent of the Port de Bales - and a category-two finish to Peyragudes.
But a minor blip 14 kilometres from the end, which saw Froome miss a curve and veer off slightly into a caravan park, proved costly for the Briton, who was unable to mount a response to attacks from Aru, stage winner Romain Bardet and Rigoberto Uran in the final 500m.
He finished 22 seconds behind Bardet and 20 adrift of Aru, who earned bonus seconds and now holds a six-second lead in the general classification.
AG2R La Mondiale rider Bardet is very much in contention - sitting 25 seconds off Aru - and Froome knows a fourth Tour title is far from a formality.
"It was a tough day. The team did an amazing job, but I just didn't have the legs at the end to finish it off, as simple as that, no excuses," Froome told France2.
"I just didn't have the legs to finish it off on the final kick. It was brutal, ramps of over 20 per cent, that's a really, really hard finish.
"I can only say congratulations to Romain Bardet for winning the stage and also to Fabio Aru for taking the yellow jersey. The race is certainly on now."