On a brutal day that saw the riders climb the legendary climb twice in the second half of the 172.5km stage, Ag2R-La Mondiale’s Riblon chased down Tejay van Garderen (BMC) on the final climb before brilliantly storming past the American with just 2km to go.
Britain’s Froome managed to finish ahead of main rival Alberto Contador (Saxo-Tinkoff) but the yellow jersey from Team Sky showed his first signs of weakness on this year's Tour, with Colombia’s Nario Quintana (Movistar) and Spain’s Joaquin Rodriguez (Katusha) distancing him in the final 5km.
Quintana and Rodriguez finished just over a minute ahead of Froome who, in turn, was just under a minute ahead of Contador to extend his lead over the Spaniard to five minutes and 11 seconds.
Tour debutant Quintana rose to third place at the expense of Contador’s team-mate Roman Kreuziger and trails Froome by 5:32 with two more mountain stages ahead of the final stage into Paris.
On a day history was made with the first ever double ascent of the famous Alpe d’Huez, it was fitting that a Frenchman stole the headlines.
Riblon overcame riding into a ditch on the treacherous Col de Sarenne descent 36km from the finish to secure the first stage win for the host nation - two years after compatriot Pierre Rolland managed to pull off the same trick in the Alpine ski resort.
The victory was also the first in the race for Riblon’s Ag2R-La Mondiale team, who have lost Maxime Bouet and Jean-Christophe Peraud to injury during a particularly challenging Tour.
“It’s incredible,” said 32-year-old Riblon. “Since the start of the race we’ve had to fight so hard. It’s the fourth time I’ve been on the attack.
“When I was small I used to watch the stages to Alpe d’Huez so I can’t believe it. When Pierre Rolland won here two years ago I was in the break and came so close. It’s incredible that I’ve finally got the win here.”
Riblon arrived at the foot of the second ascent of Alpe d’Huez in a trio alongside BMC’s van Garderen and Italian Moreno Moser of Cannondale. Moser was soon dropped before a ressurgent van Garderen, the white jersey from last year, powered clear.
The American held an advantage of 45 seconds over Riblon entering the final five kilometres but hit the wall dramatically as he neared the summit. Enjoying a second wind and buoyed by the roaring crowds, the Frenchman caught van Garderen just ahead of the 2km banner and passed him with ease to build up an insurmountable lead entering the final kilometre.
Riblon had time to savour the moment and salute the cheering crowds before crossing the line with his arms aloft to take the second Tour stage win of his career three years after his stage 14 win at Ax3 Domaines. Van Garderen came home 59 seconds down while Moser took third place at 1:27.
Quintana edged ahead of Rodriguez to take fourth place, 2:12 in arrears, before Sky’s Richie Porte led his team leader Froome over the line 56 seconds later.
Having crossed the line, Froome embraced his Australian team-mate after yet another job well done.
But it was not all plain sailing for Team Sky, with Froome having to weather the storm right from the outset of a frenetic start to the stage in Gap.
On the Cat.2 Col de Manse numerous attacks - including a stinging effort from Quintana - put Sky under instant pressure before a group of nine riders - including Riblon, van Garderen and Moser - broke clear on the descent.
The Saxo-Tinkoff team of Contador sent two men up the road - Nicolas Roche and Sergio Paulinho - on the second climb of the day, the Cat.3 Rampe du Motty.
With no one in the break posing a threat to Froome on GC (Omega Pharma-Quick Step’s Sylvain Chavanel was best-placed, more than half an hour down) the stage entered a calmer phase ahead of the back-to-back ascents of Alpe d’Huez, the first of which, 63km from the finish, the leaders reached with an advantage of seven minutes.
Van Garderen, Moser and Riblon crossed the summit together after the escapees had split apart while tackling the arduous 21 hairpin bends of the climb.
Back with the peloton, Sky controlled matters with a high tempo, only allowing riders low down in the overall standings to break clear. As such, Europcar pair Thomas Voeckler and Rolland, RadioShack’s Andy Schleck, the polka dot jersey Mikel Nieve (Euskaltel) and Dutchman Wout Poels (Vacansoleil-DCM) formed a chasing group ahead of the pack.
The narrow, twisting and highly technical descent of the Col de Sarenne was covered with a light sprinkling of rain - and Riblon overcooked a bend and rode into a shallow ditch. Van Garderen also struggled, the American youngster needing to stop for assistance with a problem with his chain.
But the major dramatics came with the peloton when Saxo-Tinkoff pair Contador and Roman Kreuziger - second and third on GC going into the stage - both attacked on the treacherous downhill.
Not wishing to take the risk, Sky refused to chase them down. It was the correct decision: the Saxo-Tinkoff pair held a buffer of just 30 seconds after the long downhill, and with Movistar driving the pace on the front of the chasing pack, they sat up in a bid to save their energy ahead of the decisive climb. The Schleck group were also reeled in moments ahead of the final climb.
With Riblon and van Garderen fighting it out for the stage win some five minutes further up the road, the fireworks exploded early in the select group of race favourites.
Froome clearly felt that attack was the best form of defence, the 28-year-old putting in one of his trademark in-the-saddle, high-cadence digs to thin out the field and drop Dutch pair Bauke Mollema and Laurens ten Dam (Belkin).
Suffering perhaps from their earlier exploits, both Contador and Kreuziger dropped back. Quintana was the only rider who could initially match Froome, although Rodriguez soon joined, followed by the ever-faithful Porte, who was greeted by the grateful Froome with a hearty hug.
With Froome looking vulnerable and tired, Porte took control of matters, driving a pace that made it impossible for Quintana or Rodriguez to attack.
But with 5km remaining, Froome suddenly signalled his team car and came to a near standstill. Porte dropped back to pick up the energy gels that his leader required - with both Sky riders picking up a 20-second penalty for an illegal refuel.
"It wasn't a huge setback," Froome said afterwards. "Richie was definitely feeling a lot better than me today. I was running out of sugars and I had to ask him to go back to the car."
Taking on food so late in stages is against the rules - but Froome shrugged off suggestions of foul play.
"Our team car has a mechanical issue earlier so we weren't able to refuel ahead of the climb," he said.
Quintana took advantage of the wobble to ride clear with Rodriguez en route to rising to a podium position in his debut Tour.
Re-energised with some much-needed sugars, Froome regrouped and rode with Porte to the finish, limiting his losses - but, crucially, also extending his lead over Contador.
It wasn’t pretty - but Froome’s lead in the general classification is now in excess of five minutes. The hug over the line between Froome and Porte said it all: despite two more tricky stages in the Alps, they have one foot in Paris.
The Tour continues on Friday with the 204.5km stage 19 from Bourg d’Oisans to Le Grand Bornand, which features five climbs and a fast downhill finish.