Simon Yates broke away to win his second Tour de France stage in a week as the Prat d'Albis saw cracks appear in Julian Alaphilippe's overall lead.
Scroll down for full stage-by-stage guide & odds
Mitchelton-Scott's Yates attacked with 8.7km left of the steep climb above Foix to follow up his victory on Thursday's stage 12 to Bagneres-de-Bigorre.
Behind, Groupama-FDJ's Thibaut Pinot attacked the reduced group of favourites and distanced Geraint Thomas, though the Welshman put in a late dig of his own to ensure he remained above the Frenchman in the general classification.
Alaphilippe retained the yellow jersey but the Deceuninck-Quick Step rider finally showed the signs of weakness which have been expected from a rider never considered a contender before the Tour began - but who will enjoy an 11th day in yellow on Tuesday.
His advantage over Thomas was cut by 27 seconds to one minute and 35 seconds.
Jumbo-Visma's Steven Kruijswijk stuck with Thomas to remain 12 seconds further back in third place but Pinot is now just a further three seconds behind in fourth place.
Yates did not need to worry about the battles unfolding behind him as he took another superb victory.
With his brother Adam's general classification hopes effectively ended in a dismal day on the Tourmalet on Saturday, Simon was free to go stage hunting again.
The 26-year-old was active in a large breakaway which included several riders licking their wounds from the previous day, with Nairo Quintana, Dan Martin, and Romain Bardet all involved.
Geschke attacked on the steep Mur de Peguere around 45km from the finish of the 185km stage from Limoux, but Yates joined him at the summit and they raced clear of the rest.
Yates then struck out for victory early on the final climb, having the power to hold both a surging Mikel Landa and the late attack of Pinot at bay.
But as much as Yates, this was also Pinot's day.
Twenty-four hours after his victory on the Tourmalet, the 29-year-old made clear it is he, and not Alaphilippe, who is France's best chance of ending a 34-year wait for a home winner on the Tour.
He launched his first move seven kilometres from the summit. Only Bora-Hansgrohe's Emanuel Buchmann and Thomas' team-mate Egan Bernal could go with him but Pinot soon shook them both off, catching Movistar's Landa and charging up the mountain to finish 33 seconds behind Yates.
With Monday a rest day, the Tour will now look forward to a fascinating final week.
The belief that Alaphilippe is to be discounted now grows, and only 39 seconds separate Thomas in second and Buchmann in sixth.
Tour de France: Stage-by-stage guide
July 23, Stage 16: Nimes to Nimes (177km)
After the second rest day comes a pretty flat stage, though with constant changes of direction on a loop starting and finishing in Nimes, the sprinters' plans could be disrupted if the Mistral wind blows.
July 24, Stage 17: Pont du Gard to Gap (200km)
And so towards the Alps, though stage 17 features none of the big passes on a day of gradual climbing up to Gap. This is the stage in the race where breaks are often allowed to stay clear as the battle for yellow rages behind, but this day has been made to be utterly unpredictable. A punchy final climb of the Col de la Sentinelle could be a springboard, while it's not inconceivable some of the sprinters survive in the main group.
July 25, Stage 18: Embrue to Valloire (208km)
There may only be five HC climbs in the entire Tour, but two of them come in a single day as the riders tackle the Izoard and the Galibier before dropping down to the finish in Valloire. The stage winner should come from whichever group first crests the Galibier - appearing for the 60th time as the Tour's most regular Alpine venue - and this could be a decisive day overall.
July 26, Stage 19: Saint-Jean-de-Maurienne to Tignes (126.5km)
Stage 19 is a leg-sapping day designed to test the resolve of anyone planning a late move in the general classification. The Iseran - at 2,770m the high point of the Tour - comes in the middle of the stage and casts a long shadow before the shorter climb to Tignes offers an opportunity for attacks.
July 27, Stage 20: Albertville to Val Thorens (130km)
The last chance saloon for GC hopefuls does not look an inviting one. The final climb of the Tour is the 33.4km HC slog up to Val Thorens. If ever there was a Tour designed for a true climber, one with only a handful of time trial kilometres and four HC climbs in the last three days could be it.
July 28, Stage 21: Rambouillet to Paris Champs-Elysees (128km)
There are two big questions on the final day. Will the yellow jersey winner choose a few glasses of champagne or, as Geraint Thomas did last year, prefer a beer or two as they celebrate along the roads of the Parisian suburbs? And which sprinters will have survived the mountains in order to battle it out on the Champs-Elysees?