Aru suffers on Galibier as Martin moves up but Froome extends
Chris Froome extended his lead in the yellow jersey as former ski jumper Primoz Roglic sailed over the Tour de France's highest mountain to win stage 17 into Serre-Chevalier.
Roglic went solo six kilometres from the top of the Col du Galibier and crested the summit with a 90-second lead before racing down the 28km descent to the finish.
Most of the main contenders were in the second group on the road, 73 seconds further back.
Colombian Rigoberto Uran took second place for six bonus seconds while Froome pipped French rival Romain Bardet to the line for the final four.
That sees three-time Tour winner Froome extend his lead, with Uran and Bardet both now 27 seconds back.
Italian national champion Fabio Aru had been dropped on the Galibier and conceded 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 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.
"I didn't have the legs in the Pyrenees," said Froome, who had lost yellow on stage 12 to Peyragudes before reclaiming it 48 hours later.
"But now I feel strong. I've also got to look towards tomorrow because it's a summit finish on Izoard and I'll have to see how the legs are."
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 Aru.
Simon Yates, wearing the best young riders' white jersey, also struggled, finishing three minutes and 14 seconds behind Roglic.
That saw him concede 90 seconds to rival Louis Meintjes in the battle for white, though Yates still has a cushion of two minutes 28 seconds.
"I think I did well to limit my losses really," Yates said. "I rode full gas. I wasn't strong enough, there was quite a lot of wind about and I got tailed off towards the end.
"I really fought hard to stay on because I knew it was a long way to the finish, but I couldn't do anymore."
LottoNL-Jumbo's Roglic emerged from a sizeable breakaway, partly powered by an aggressive ride from veteran Alberto Contador.
Roglic 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 the way.
Martin was the first to attack before AG2R La Mondiale's Bardet put in a dig. It soon became clear that Aru was struggling and they took it in turns to ensure the Astana man was left behind.