Simon Yates attacked breakaway companion Simon Geschke (CCC) nine kilometres from the summit finish, holding off a late charge from Thibaut Pinot.
"I'm very proud of what I did there," he said. "It was an extremely hard day, really from the start to the finish. I raced the way I liked to, which is to try and be aggressive and I managed to pull it off, so I'm really happy."
"I was worried that they were catching me. I really needed to go at that point because I wasn't confident that we had a big enough gap. So that's what I did."
Behind Yates, the GC battle fired up as Thibaut Pinot (Groupama FDJ) unleashed a massive attack on the final climb, six kilometres from the finish with only Emanuel Buchmann (BORA-hansgrohe), Egan Bernal (Team Ineos) and Julian Alaphilippe (Deceuninck-QuickStep) able to follow.
Pinot continued to ride the front hard, launching repeated strikes, first cracking the man in yellow and soon shelling the others. The Frenchman crossed the line second after powering past Mikel Landa who earlier launched an attack of his own on the Mur de Peguere, 40kms from the finish and bridged to his Movistar team mates in the day's break.
Buchmann crossed the line fourth, leading home Bernal as Geraint Thomas attacked Alaphilippe a little further down the hill to grab some time in the final kilometres, the Welshman unable to shake Steven Kruijswijk (Jumbo-Visma) and Alejandro Valverde (Movistar).
Alaphilippe finished over a minute down on Pinot, but still enjoys a healthy one minute and 35 second lead over Thomas.
Yates was part of a large 36-strong group that collaborated after the ascent of the Col de Montsegur and several earlier skirmishes. Big GC names who squandered time on the Tourmalet including Nairo Quintana (Movistar), Romain Bardet (Ag2R) and Dan Martin (UAE) were allowed away alongside 2014 Tour de France champion Vincenzo Nibali (Bahrain Merida).
On the category 1 climb of the Port de Lers, the breakaway fractured, with the strongest climbers coming to the fore and slimming the front riders down to a select few. CCC's Simon Geschke slipped away on the flat, building a decent advantage over the breakaway, before reaching the penultimate climb of the stage. Yates made it over to the German, the pair combining on the descent to take a minute's lead with 20 kilometres to go.
Meanwhile, Landa broke free from the peloton to reach his Movistar team mates, quickly building a minute's lead on the yellow jersey group as they approached the final category 1 climb of Prat d'Albis. Yates gapped Geschke with his assault on the final climb, Landa continuing his surge behind to clean up the remnants of the breakaway including the German.