Britain's Geraint Thomas has extended his Tour de France lead and Chris Froome's hopes of a record-equalling fifth title are hanging by a thread after the defending champion was dropped in the finale of 17th stage.
Colombian Nairo Quintana attacked at the foot of the final climb up to the Col du Portet to snatch a prestigious victory 28 seconds ahead of Ireland's Dan Martin on Wednesday, but all eyes were on yellow jersey holder Thomas and four-time champion Froome.
Froome, attempting a first Giro d'Italia-Tour double in 20 years, cracked 2km from the finish when Dutchman Tom Dumoulin attacked, and slipped from second to third overall.
Thomas, who took third place 47 seconds off the pace, now leads Dumoulin by 1:59 and Froome by 2:31 and is widely expected to hand Team Sky their sixth Tour title in seven years.
Froome's hopes are hanging by a thread after he crossed the line 1:35 off the pace as he does not seem to have the legs to overturn the deficit and might not be allowed to by his team after Thomas emerged as the clear leader in the British squad.
Froome's last hope appears to rest on whether Thomas cracks in the last mountain stage on Friday, which seems unlikely after the Welshman proved stronger in all three summit finishes.
Organisers had set up a grid start at the foot of the first climb but, while it often brings excitement in Formula One, it was a forgettable moment here, with the peloton being well-organised after a few hundred metres.
Team Sky controlled the pace as several riders attacked in the Montee de Peyragudes, a 14.9-km ascent at an average gradient of 6.7 per cent.
Among them was France's Julian Alaphilippe, who strengthened his lead in the mountain classification.
Romain Bardet's AG2R-La Mondiale team upped the pace in the second climb, the Col du Val Louron Azet (7.4km at 8.3 per cent), to shake up the peloton and hoped to drop several Team Sky domestiques but they failed.
Those tactics even backfired in the last ascent, a 16km effort at an average gradient of 8.7 per cent, as the Frenchman cracked and had no team mate left to help him.
At the foot of that last effort, Quintana jumped away in impressive style followed by Martin who quickly found himself unable to sustain the pace of the twice Tour runner-up.
With 14km left, Slovenia's fourth-placed Primoz Roglic also attacked and was immediately followed by Froome. The duo opened a small gap before being reined in by the yellow jersey group thanks to the work of Dumoulin protecting his podium place.
Froome started to suffer and he was briefly distanced 2.5km from the top on another Roglic attack.
Dumoulin's acceleration then buried Froome, who even had trouble holding his team mate Egan Bernal's wheel.
Thomas finally made his point, accelerating with about 400 metres left to drop Dumoulin and Roglic.
World champion Peter Sagan suffered a spectacular crash in the descent from the Col du Val Louron but remounted his bike and made the time cut, his jersey torn to pieces.