Primoz Roglic took a giant leap towards a maiden Tour de France title when he extended his overall lead after the 17th stage, an unforgiving mountain rollercoaster dominated by Colombian Miguel Angel Lopez.
Lopez prevailed at the end of the 170-km trek from Grenoble, 15 seconds ahead of Roglic, who extended his advantage over fellow Slovenian Tadej Pogacar to 57 seconds, four days before the final parade in Paris.
Roglic was only second best at the top of the Col de la Loze, an unforgiving 21.5km ascent at an average gradient of 7.8%, but it was enough for the Jumbo-Visma rider to tighten his grip on a race he has controlled from the start.
Australia's Richie Porte finished fifth to move up to fourth on the general classification, but trails Roglic by 3.05 minutes.
"It was again a good day for us," Roglic told reporters.
"Of course, I always want to win but I gained some time and I saw that others had problems. I knew I could gain time today and that's what we did."
Lopez, twice a Vuelta stage winner, jumped away from the leading group 3.5km from the finish and appeared to defy gravity in the finale as the road rose up to 25 per cent under threatening skies, 2.3 metres above sea level.
The 21-year-old Pogacar now faces an uphill battle to unsettle Roglic, who suffered but was never really threatened.
Lopez moved up to third in the standings, 1:26 off the pace, ahead of another gruelling mountain stage on Thursday, over 175km between Meribel and La Roche sur Foron.
"For the moment I'm happy that I won this stage, it's the biggest win of my carer and I'd be happy with a podium finish," Lopez told reporters.
Another Colombian, Rigoberto Uran, had a bad day and slipped down to sixth from third.
The stage started without defending champion Egan Bernal, who pulled out because of back pains, with his Ineos Grenadiers team saying he would focus on new goals this season.
The British team shone on the road, however, with Richard Carapaz the last man standing in the day's breakaway, although the Giro d'Italia champion was caught by the main contenders when Lopez upped the pace.
Spaniard Mikel Landa's Bahrain-McLaren team set a high pace in the Col de la Madeleine, hoping to shake off a few of Roglic's team mates.
While the move meant that twice runner-up Nairo Quintana was dropped, the Jumbo-Visma domestiques stayed with the group and the Dutch outfit were in bigger numbers in the finale.
Three breakaway riders - Julian Alaphilippe, Carapaz, Gorka Izagirre - reached the bottom of the Col de la Loze with a 1:50 advantage, but it was clear it would not be enough on the toughest climb of this year's Tour.
Carapaz, however, capitulated when the top guns started the battle.
Lopez opened a gap over Roglic and Pogacar and never looked back, earning his Astana team their second stage win in this year's Tour.
Roglic did not respond at first, but he then accelerated to drop Pogacar, who will need to take risks in the last mountain test on Thursday if he is to beat the odds.
French president Emmanuel Macron followed part of the stage on the road, where hundreds of fans gathered despite the COVID-19 outbreak.
"It's extremely important to show that we can live with the virus," said Macron.
Australian Associated Press