Chris Froome showed rare signs of weakness despite extending his overall lead on the Tour de France in the 18th stage, won after two epic ascents of l'Alpe d'Huez by France's Christophe Riblon.
The Briton cracked with less than five kilometres to go on the second trip up the mountain's 21 hairpin bends but sent team mate Richie Porte to fetch some food and the Australian nursed Froome to the finish 3:18 behind Riblon, who became the first Frenchman to win a stage in this year's race.
"I was running low on sugars," Froome told reporters.
"I am thankful to have Richie with me he did a super, super job."
Froome and Porte were each given a 20-second penalty for illegal feeding as organisers said the riders were not allowed to take food or liquid from the team car during the final 6km of the stage.
However, Froome still leads Alberto Contador by 5:11, after the Spaniard struggled to keep the pace in the final ascent following a bold but brief attack on the treacherous descent from the Col de Sarenne.
"I was not well. It was very hot and humid. It was a bad day," Contador, Tour winner in 2007 and 2009, told reporters. "But I managed to limit the damage."
PODIUM FINISH
Contador, who was 11th on the stage, 57 seconds behind Froome, heads Colombian Nairo Quintana by 21 seconds overall after the Movistar climber, targeting a podium finish in Paris, took fourth place.
"I've never been as happy as I am today - I've gone from sixth to third overall, I've taken one minute out of Froome's lead... I didn't realise that he had some problems but my timing was ideal. I had a bit of luck," Quintana told reporters.
"Tomorrow and the next day are very hard stages so have to recover quickly and get ready for the challenges that lie ahead... but I accept that winning this Tour is no longer an obtainable objective - the gap to Froome is too big."
Predictably, Contador attacked in the descent from the Col de Sarenne along with team mate Roman Kreuziger, only for the pair to sit up after opening a gap of just 20 seconds over Froome and co.
Quintana's Movistar team drilled it at the front of the favourites' group as Contador changed bike because of a minor technical problem.
The Spaniard was left once again with no answer to Froome's acceleration but the British rider also struggled by the end as Joaquim Rodriguez and Quintana sped away.
DEMANDING STAGES
"Throughout the entire race we've said that we wanted the overall win and we were not afraid of risking it all to get it and so we did today," Contador's Team Saxo Tinkoff sports director Fabrizio Guidi said.
"But the legs just weren't good enough to finish it off. We can be disappointed and everyone's a lot wiser after the stage but if you never try, you never win.
"We're still competing for the top positions and there are still two demanding stages to go. Now, it's time to keep focused, remain calm, recover and try again tomorrow," Guidi added.
Riblon, part of the early breakaway, caught American Tejay van Garderen with just over one kilometre left and, after fighting his way through waves of delirious fans overflowing on to the road, finished 58 seconds ahead as AG2R found something to cheer about a day after their leader Jean-Christophe Peraud crashed out of the race.
It was a second career Tour win for Riblon and came three years to the day after he claimed the Ax-3-Domaines stage.
"It's l'Alpe d'Huez, I was watching this climb on TV when I was not even 10," Riblon told reporters.
"Two years ago I was in the break with Pierre Rolland when he won and I told myself I had let a huge opportunity slip.
"When I saw Van Garderen with three kilometres left, I told myself it was a fantastic chance so when I caught him I left him no hope that he could contest the win."
Friday's 19th stage takes the peloton over 204 kilometres with four demanding climbs on the menu between Bourg d'Oisans and Le Grand Bornand.
"Tomorrow is possibly the toughest day of the Tour de France," said Froome.
