Geraint Thomas took a giant stride towards claiming his first Grand Tour title and Chris Froome's reign as Tour de France champion all-but ended on a brutal stage 17 won by Nairo Quintana.
A gruelling 65-kilometre ride from Bagneres-de-Luchon was expected to be pivotal in the general classification battle and Thomas passed another major test with a brilliant climb, increasing his lead to one minute, 59 seconds.
Thomas burst away from Primoz Roglic and Tom Dumoulin at the end of a punishing final ascent of the Col de Portet to finish third behind Quintana and Dan Martin in Saint-Lary-Soulan.
Dumoulin moved above Froome into second place in the GC after the four-time champion was dropped on the last of three lung-bursting climbs in the Pyrenees on Wednesday.
While Froome faded late in the day, the indefatigable Thomas strengthened his grip on the yellow jersey and is the red-hot favourite to be on the top step of the podium in Paris on Sunday.
Froome trails his Team Sky team-mate by two minutes and 31 seconds, leaving his chances of claiming a record-equalling fifth Tour title looking slim at best, while Romain Bardet dropped to eight in the GC standings after feeling the effects of a punishing day.
Quintana had no such trouble despite two early stops due to mechanical issues, producing a brilliant long-range attack to secure a first Tour stage win since 2013 at the highest point of this year's race.
Astana's Tanel Kangert moved clear following a grid start in Bagneres-de-Luchon and was only caught by Rafał Majka and Quintana 8.5km from the finish.
Movistar's Quintana went solo with 6.4km to go and never looked back, winning by 28 seconds from Martin as the GC battle unfolded behind them on the last 16km ascent.
SAGAN CRASHES, ALAPHILIPPE HITS THE HEIGHTS AGAIN
Peter Sagan, already mathematically assured of a record-equalling sixth green jersey, finished within the time limit after suffering a crash 38km into the stage and will undergo medical examinations.
The sprint king crossed the line over 26 minutes after Quintana with a torn jersey before limping off looking in pain.
King of the Mountains leader Julian Alaphilippe had another fruitful day, chasing Kangert up the first climb of the Category 1 Montee de Peyragudes to add eight points to his tally.
The Quick-Step Floors rider surged up the Col de Val Louron-Azet to claim another 10 points in the polka dot jersey and now leads Warren Barguil by 73.
STAGE 17 RESULT
1. Nairo Quintana (Movistar) 02:21.27
2. Dan Martin (UAE Team Emirates) +0.28
3. Geraint Thomas (Team Sky) +0.47
4. Primoz Roglic (Lotto NL-Jumbo) +0.52
5. Tom Dumoulin (Team Sunweb) +0.52
1. Geraint Thomas (Team Sky) 62:49.47
2. Tom Dumoulin (Team Sunweb) +1.59
3. Chris Froome (Team Sky) +2.31
1. Peter Sagan (Bora-Hansgrohe) 452
2. Alexander Kristoff (UAE Team Emirates) 170
3. Julian Alaphilippe (Quick-Step Floors) 134
King of the Mountains
1. Julian Alaphilippe (Quick-Step Floors) 140
2. Warren Barguil (Team Fortuneo-Samsic) 73
3. Geraint Thomas (Team Sky) 54
A 171km flat stage 18 from Trie-sur-Baise to Pau, with a bunch sprint expected on a descending finale.