Chris Froome put the Tour de France almost out of sight of his rivals after winning the stage 18 uphill time trial by 21 seconds.
Dutchman Tom Dumoulin, a time trial specialist, finished in second with Froome’s nearest overall challenger, Trek Segafredo’s Bauke Mollema, finishing 1min25s down in 17th place.
Froome’s compatriot, Adam Yates, finished one place ahead of Mollema to take two seconds back from his Dutch rival and hold on to third place in the overall standings.
Dumoulin had set the outstanding time of the day until Froome – who went off last – blasted past his time on the 17km climb to Megeve.
Froome now leads the general classification by 3min52s from Mollema, with just two mountain stages to go before Sunday’s sprint stage into Paris and along the Champs Elysees.
"I really didn't expect to beat Tom Dumoulin – pacing was key,” said the two-time Tour winner.
“I started steady and controlled the first part and then really went for it over the top. I'm really happy with that.
"We have two more big days to come, hopefully I didn't leave too much out on the road today. But I'll be giving it everything to get the job done.
"Tomorrow is a very tricky stage with tricky descents and there is talk of thunderstorms. We cannot relax and switch off."
Orica BikeExchange’s Yates, who is now 4min16s behind Froome overall, may have closed the gap on Mollema but his buffer to fourth-placed Nairo Quintana was cut to 21s, after Movistar’s Colombian finished 13s ahead of the white jersey-wearer.
"I was strong at the first time check and then my legs fell off. I did the best I could. I didn't lose too much time. I've got two more days to fight and we'll fight for every second,” said 23-year-old Yates.
"It's going to be a big fight for the podium."
Tomorrow sees the riders tackle a 146km route from Albertville to Saint-Gervais Mont Blanc, over four categorised climbs including the 1,732m Montee de Bisanne.
Froome’s rivals will need to attack early in order to gain enough time back on the Team Sky rider, with the ten kilometre climb to the finish line, expected to be decisive.