Matej Mohoric celebrated a solo victory as the longest stage of the Tour de France in 21 years delivered another shake-up of the fight for yellow.
Mathieu van der Poel extended his lead in yellow after getting into a 29-man breakaway which tested the resources of defending champion Tadej Pogacar and his UAE Team Emirates squad to the limit over the course of 249 gruelling kilometres between Vierzon and Le Creusot.
Pogacar came home as part of a much-reduced group of contenders more than five minutes after Mohoric and three and a half minutes after Van der Poel, dropping from second to fifth overall, but after a wild day it is hard to say if his status as Tour favourite is weakened or enhanced.
Primoz Roglic, seen as Pogacar's main rival, appears out of the game, dropped on the climbs late in the day and conceding almost four minutes to the other contenders.
Geraint Thomas, another victim of the crash-strewn stage three, was also dropped late on just as his Ineos Grenadiers team-mate Richard Carapaz was attacking out of the reduced peloton, but the Welshman was able to ride back on as Carapaz was reeled in before the line, with no time gaps between them.
But Van der Poel's remarkable debut Tour continues as the Dutchman now leads by 30 seconds from Roglic's Jumbo-Visma team-mate Wout van Aert, with Kasper Asgreen and Mohoric moving up to third and fourth respectively after their day in the break.
Pogacar sits three minutes 43 seconds down in fifth, still the best placed of the main favourites, but there will be questions over his supporting cast after they were made to chase the breakaway for the bulk of five and a half testing hours in the saddle.
The news was far worse for Roglic, however. The Jumbo-Visma rider's time trial performance on Wednesday suggested he had survived the impact of a crash earlier in the week, but the pain was surely felt here as he lost almost four minutes and dropped down to 33rd place overall.
Roglic went backwards on the Signal d'Uchon as gradients hit 18 per cent. Thomas, nursing the shoulder he dislocated on Monday, would also be distanced close to the summit before recovering.
"It was quite a big crash," Thomas said. "It was easy to talk myself into it, 'I'm OK, blah, blah, blah,' but it still takes a lot out of you, as you saw with Roglic.
"I didn't want to go full 100 per cent (on the climb). I was still in contact with 200 metres to go so I tried to pace myself a bit and I came back was it was OK in the end.
"I'm suffering but hopefully I'll feel better soon, though maybe not with the next two days to come (in the mountains)."
There will be no let-up on Saturday with the first true mountain stage going immediately uphill on the road to Le Grand-Bornand, suggesting anyone who struggled here will have only more pain to come.
The prospect of two days in the Alps and a daunting distance in front of them might have persuaded the peloton to take it easy, but it proved the opposite as a huge fight to get in the break kept the pace up for the first 50km.
Mark Cavendish, still celebrating the second of his stage wins on Thursday, got into the group and used the intermediate sprint to strengthen his grip on the green jersey before dropping back as the battle for stage honours began.
Mohoric was at the forefront of the attacks, at first seeking king of the mountains points but realising after the summit of the Croix de la Liberation inside the final 40km that much more was on offer.
As the Slovenian soloed to victory, Van der Poel and Van Aert darted off the front of the remaining break, glued together as they raced the last 10 kilometres to pick up more time on their rivals.
"It was pure racing," Van der Poel said. "I've not seen this often, a race like this with 250km in a Grand Tour, so I think everybody will sleep well tonight."
