Vincenzo Nibali took advantage of a Chris Froome mechanical problem to take one minute and 14 seconds back and move into fourth place in General Classification as the reigning champion won stage 19.
Astana's Nibali moved to within one minute and 19 seconds of getting a place on the podium as Movistar's Nairo Quintana also gained 30 seconds on Froome to close the gap to 2'38" with one day left in the Alps before the finale in Paris on Sunday.
Fourth at the start of the day, Team Sky's Geraint Thomas lost a massive amount of time, slipping down to 15th overall, 27'24" behind the Yellow Jersey.
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Froome explained the moment he knew he had a problem: "I felt like my back wheel locked up so I had to stop to get the stone out and that was the moment Nibali decided to do his attack. I'm sure he knew I was in trouble and I don't think it's sportsman like at all."
In respect of the stage as a whole Froome said: "It was such a massive day today and the racing was on from the start. It was a tough day for the team and especially G [Geraint Thomas]."
Nibali appeared to look round at Froome three times before attacking on the Col du Glandon, after he noticed the Team Sky rider had some sort of mechanical problem, and made the move stick working with Europcar's Pierre Rolland to open the gap to over two minutes to the chasing Yellow Jersey group at one point on the descent.
Realising he was losing time too Quintana made his move on the final climb of the day up to the finish in La Toussuire, leaving GC rivals Alberto Contador, Movistar team mate Alejandro Valverde and Robert Gesink trailing and Froome struggling to keep up.
Stage 18 winner Romain Bardet is now within one stage of winning the King of the Mountains classification after former holder Joachim Rodriguez cracked on the Col du Glandon. The AG2R rider has 90 points to Froome's 87 and Katusha's Rodriguez with 78, with the winner of the Polka Dot Jersey probably being the one who can top Alpe d'Huez first tomorrow.
The racing was frantic from kilometre 0, and with the peloton fragmented following fierce attacks on the first climb of the day, the Col du Chaussy, Europcar's French rider Pierre Rolland went on a solo venture up the Col du Glandon in an attempt to be the first to summit the Hors Categorie, Col de la Croix de Fer.
Astana turned the screw as the Yellow Jersey group crawled up the Col du Glandon, splintering the ranks of riders and putting Froome and Team Sky under real pressure.
Neither Richie Porte nor Geraint Thomas could keep up with the pace of the Kazakhstan based team and were soon dropped, leaving just Wouter Poels and Froome left from Team Sky to defend the Astana attack.
On the Croix de Fer Froome then suffered a disaster, a mechanical issue, or more specifically a stone in his rear brake caliper. Nibali looked behind him to check on Froome at least three times to see what was wrong and decided to take the opportunity to attack, pushing an advantage out to 40 seconds within a few minutes.
Rolland crested the Croix de Fer to gain 25 points in the King of the Mountains classification, with a one minute gap to Nibali.
After the fast, tricky and technical descent down the Croix de Fer, Rolland began the category two climb, Col du Mollard, with a 54 second advantage over Nibali and a further 1'25" back to the chasing Yellow Jersey group.
In the overall standings Nibali was still over 6' behind Froome, leap-frogging Thomas, Contador and Gesink, but nobody seemed in much of a hurry to close down the Italian national champion.
Going over the Col du Mollard Nibali caught Rolland, and they looked to join forces on the descent before the final climb up to La Toussuire.
Bardet gave chase to Nibali and Rolland, however, had a problem with his gearing selection and, with his team car stuck behind the Yellow Jersey group, was unable to get a bike change so had to make do with only his lower gears, slowing the Frenchman considerably.
Bardet eventually got a bike change after waiting many minutes for his team car but had to be content with sitting on the back of the Yellow Jersey group.
As he did so, Nibali and Rolland started the final climb of the day up to La Toussuirre - the very same climb where Froome infamously dropped Bradley Wiggins in the 2012 Tour. In a bid to reclaim some of the time lost to Nibali, and take back fourth, Contador made a bike change too.
16km to the summit and Nibali attacked Rolland, looking to gain even more time on Froome before tomorrow's ascent of Alpe d'Huez, and within 2km had opened a 49 second buffer to Rolland and a 2'07" gap to the Yellow Jersey group.
Rolland was caught by the Yellow Jersey group 7km from the finish, which would have been disappointing for the Europcar rider, who had led the race for the majority of the stage.
Poels' job was done with 5km to go and dropped off the back of the Yellow Jersey group. At the same time Quintana launched an attack and was swiftly followed by Froome, Contador and Valverde.
No sooner had the attack began than Contador and Valverde both cracked, leaving just Froome to chase the little Colombian and try to bring back some of the time lost to Nibali. Froome couldn't stay with Quintana either though, allowing the Movistar rider to gain a 40 second gap at the finish line.
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