Geraint Thomas seized full control of the Tour de France as he extended his lead while Sky team-mate Chris Froome slipped back to third overall on stage 17.
While Movistar's Nairo Quintana won atop the Col du Portet to move back into the top five, Froome lost contact with the main group of contenders around two kilometres before the summit.
As he went backwards, Thomas stuck on the wheels of Tom Dumoulin and Primoz Roglic - who started the day third and fourth overall - before racing clear in the final few hundred metres to take third place on the day behind Irishman Dan Martin.
Martin moves up to ninth overall, though he remains nearly a minute and-a-half down on eighth place Romain Bardet, who had a bad day on Wednesday.
"The last few days I felt really good, which is a good sign," said Martin.
"I planned to go earlier, but it's okay saying to attack early but with so much wind on those early climbs it wasn't worth it. It was up to Sky to set the tempo at the start of the last climb and they just let me away.
"Nairo came with me and went past me. I tried to ride it like a time trial, but the altitude got me in the end. I was trying to keep him at ten, 15 seconds because over the last 500 metres I'd think I could close that gap, but he was the better rider."
Welshman Thomas picked up four seconds on the road and four bonus seconds for third.
"He now leads by one minute and 59 seconds from Team Sunweb's Dumoulin, with Froome two minutes and 31 seconds back in third.
Froome finished 48 seconds behind Thomas on the day, and as he warmed down on rollers, he sounded ready to put aside his pursuit of a record-equalling fifth Tour crown in order to support his team-mate.
"It was a tough day, an intense day, but I've got no regrets," Froome said. "(Thomas) has ridden such an amazing race, he deserves to be in yellow and fingers crossed he holds it now until Paris."
Asked about riding in service of Thomas as the defending champion, he added: "That's professional cycling, that's what a team is all about.
"I'm happy just to be in the position I'm in. I've won the last three Grand Tours I've done now. It's certainly been a tough build-up for me but I'll still fight for the podium and obviously we want to see (Thomas) up there in yellow.
"I just didn't have the legs. I think he's got an almost two-minute lead on Dumoulin which is a pretty comfortable buffer.
"He looks pretty strong and I imagine he'll be able to finish it off. We just need to look after him now for these next few days."
While Froome was dropped in the finale, at one stage it looked like going the other way as he covered a move by LottoNL-Jumbo's Roglic while Thomas hung back on the wheel of Dumoulin, leaving the Dutchman to close the gap.
Quintana, who started the day more than four minutes back in the general classification, hauled himself back into the top five with the second Tour stage win of his career.
The Movistar rider attacked close to the foot of the 16km climb as he followed a move launched by Martin, though he quickly shook off the UAE Team Emirates rider who was left to pursue him alone.
Quintana's victory came despite two early mechanicals on a 65km stage - the shortest road stage in the Tour for over 30 years - that was designed to be explosive from the start.
A Formula One-style grid start turned into something of a damp squib but the final climb appears to have provided one of the decisive moments of the Tour.