Chris Froome moved even further clear overall.
Riblon crossed the line all alone with arms aloft after catching American Tejay Van Garderen two kilometres from the finish and racing away to win by 58 seconds.
The 32-year-old won despite being the only man to come to grief on the much-feared descent of the Col de Sarenne today, riding into a ditch before clambering back to the road.
Froome showed his first signs of struggling all Tour when he called desperately for an energy bar in the final five kilometres, but he nevertheless took almost a minute out of closest rival Alberto Contador, who suffered even more.
Froome finished seventh alongside Richie Porte in sixth, and although he surrendered a little over a minute to Nairo Quintana, he can be happy to have seen his overall lead increase once again on such a dangerous stage.
With the forecast thunderstorms staying away, Contador had attacked Froome on the feared descent of the Col de Sarenne which linked the two ascent of the Alpe.
Throwing caution to the wind, he and team-mate Roman Kreuziger pulled 20 seconds clear, but could not stretch it any further and instead sat up to be caught before the second climb.
No sooner had he been caught than he had dropped back for a bike change although he was back with Froome on the early part of the climb.
Movistar were leading the little pack in a bid to put Quintana in a winning position but it was Froome who kicked a little over 11 kilometres from the summit.
Quintana followed but Contador could not and quickly began to fall away.
Up ahead, Van Garderen had dropped Riblon and Moreno Moser to go clear alone, building a lead of 40 seconds.
However, Riblon began to close in before the five kilometre mark, which is also where the late drama would come for Froome.
The race leader has looked imperious on these mountain climbs up to now, but this time he was frantically waving to the team car as they rounded a bend.
But rather than drop back himself, Porte went, apparently to pick up an energy bar for his team-mate.
Race rules bar riders from taking food from the team car for themselves within the final five kilometres, and it remains to be seen how the commissaires will view the incident.
Froome looked like a man out of energy, and Contador was able to claw back 20 seconds in those final kilometres to limit his losses.