Primoz Roglic won stage 17 of the Tour de France to Serre-Chevalier as Chris Froome finished third to retain the yellow jersey.
Slovenian Roglic led by 90 seconds at the top of the Galibier and the former junior world ski jump champion showed no fear on the long fast descent to the finish to win by 73 seconds.
Rigoberto Uran finished second to take six vital bonus seconds in the close battle for yellow, with Froome taking four for this third place by pipping Romain Bardet to the line.
Three-time Tour winner Froome now leads by 27 seconds, with Colombian Uran and Frenchman Bardet tied for second place.
Froome, Uran and Bardet finished in the second group on the road, but Italian national champion Fabio Aru had been dropped before the top of the Galibier, conceding more than 30 seconds to lose his grip on second place overall.
Aru dropped down to fourth, 53 seconds off yellow. Froome’s team-mate Landa sits fifth while Irishman Dan Martin of Quick-Step Floors moved up one place to sixth with a battling performance 24 hours after he was caught out in crosswinds and gave up two spots in the general classification.
For Team Sky’s Froome it was a case of job done as he ticked off the first of two Alpine stages which could decide this year’s Tour even before Saturday’s time trial in Marseille.
The main rivals had all tested each other on the long gradual climb of the famed Galibier, the highest point on this year’s Tour with a summit at 2,642m, but they could only shake off Astana’s Aru.
Bury’s Simon Yates, wearing the best young riders’ white jersey, also struggled, finishing three minutes and 14 seconds behind Roglic.
That saw the Orica-Scott rider concede 90 seconds to South African rival Louis Meintjes in the battle for white, though Yates still has a cushion of two minutes and 28 seconds.
Roglic emerged from a sizeable breakaway, partly powered by an aggressive ride from veteran Alberto Contador.
The LottoNL-Jumbo rider attacked solo six kilometres from the summit of the Galibier, the second of two hors categorie climbs on a testing day, and made it pay to deliver Slovenia’s first Tour de France stage win.
Only 29 seconds had separated the top four on the general classification at the start of the day and they closely marked each other all day.
Martin was the first to attack and after he was reeled in, AG2R La Mondiale’s Bardet took his turn. It soon became clear that Aru was struggling and they took it in turns to ensure the Astana man was left behind.
Froome still had team-mate Landa with him as they tried to set up the sprint finish for the bonus points, but although Froome made it around Bardet he could not beat Cannondale-Drapac’s Uran to the line.
There had been major news early on the stage when Marcel Kittel, wearing the points leaders’ green jersey and winner of five stages in this Tour, was forced to abandon the race following a crash.
The German had hit the deck in a crash that also brought down Warren Barguil in the polka dot jersey and British national champion Steve Cummings, who both recovered.
Quick-Step Floors’ Kittel began the day with a 29-point lead over Australian Michael Matthews in the points classification, but had already seen that cut to just nine as his Team Sunweb rival won the day’s intermediate sprint.
The points classification had been won by world champion Peter Sagan in each of the previous five Tours but the Slovakian was disqualified at the end of stage four to leave the competition open this year.
Kittel was not alone in abandoning on one of the toughest days of the entire Tour.
Frenchman Thibaut Pinot, a contender in previous years, also quit to leave his FDJ team with just three of their nine original riders left in the race.
British sprinter Dan McLay of Fortuneo-Oscaro abandoned on the Col du Telegraphe.
Leading positions after Stage 17 (Le Murre – Serre Chevalier- 183km):
1 Primoz Roglic (Slo) Team LottoNl-Jumbo 5hrs 07mins 41secs
2 Rigoberto Uran (Col) Cannondale-Drapac at 1min 13secs
3 Christopher Froome (Gbr) Team Sky
4 Romain Bardet (Fra) AG2R La Mondiale
5 Warren Barguil (Fra) Team Sunwebat same time
6 Mikel Landa (Spa) Team Sky at 1min 16secs
7 Daniel Martin (Irl) Quick-Step Floors at 1:43secs
8 Alberto Contador (Spa) Trek-Segafredo at 1:44secs
9 Louis Meintjes (Rsa) UAE Team Emirates
10 Fabio Aru (Ita) Astana Pro Team
11 Mathias Frank (Swi) AG2R La Mondiale at same time
12 Darwin Atapuma (Col) UAE Team Emirates at 1min 59secs
13 Serge Pauwels (Bel) Dimension Data at 3mins 10secs
14 Simon Yates (Gbr) Orica-Scott at 3mins 14secs
15 Daniel Navarro (Spa) Cofidis, Solutions Credits at 5mins 07secs
16 Damiano Caruso (Ita) BMC Racing Team
17 Ondrej Cink (Cze) Bahrain-Merida at same time
18 Amael Moinard (Fra) BMC Racing Team at 6mins 00secs
19 Alexis Vuillermoz (Fra) AG2R La Mondiale
20 Brice Feillu (Fra) Team Fortuneo – Oscaro at same time
General Classification after Stage 17:
1 Christopher Froome (Gbr) Team Sky 73hrs 27mins 26secs
2 Rigoberto Uran (Col) Cannondale-Drapac at 27secs
3 Romain Bardet (Fra) AG2R La Mondiale at same time
4 Fabio Aru (Ita) Astana Pro Team at 53secs
5 Mikel Landa (Spa) Team Sky at 1min 24secs
6 Daniel Martin (Irl) Quick-Step Floors at 2mins 37secs
7 Simon Yates (Gbr) Orica-Scott at 4mins 07secs
8 Louis Meintjes (Rsa) UAE Team Emirates at 6mins 35secs
9 Alberto Contador (Spa) Trek-Segafredo at 7mins 45secs
10 Warren Barguil (Fra) Team Sunweb at 8mins 52secs
11 Damiano Caruso (Ita) BMC Racing Team at 10mins 03secs
12 Nairo Quintana (Col) Movistar Team at 12mins 54secs
13 Alexis Vuillermoz (Fra) AG2R La Mondiale at 20mins 36secs
14 Mikel Nieve (Spa) Team Skyat 21mins 16secs
15 Emanuel Buchmann (Ger) Bora-Hansgroheat 27mins 06secs
16 Brice Feillu (Fra) Team Fortuneo – Oscaroat 33mins 32secs
17 Serge Pauwels (Bel) Dimension Dataat 34mins 47secs
18 Carlos Betancur (Col) Movistar Teamat 35mins 23secs,
19 Tiesj Benoot (Bel) Lotto Soudalat 37mins 46secs
20 Guillaume Martin (Fra) Wanty – Groupe Gobertat 43mins 53secs