Grenoble fought back from 18 points down to claim a shock 25-24 victory over Top 14 heavyweights Toulon at the Stade des Alpes on Saturday.
The results sees Toulon relinquish the championship lead to Clermont with one round left to play although the leading duo have both already qualified for the semi-finals.
Toulon led 21-6 at half time and 24-6 early in the second half before succumbing to an almighty comeback from the hosts, who hadn't won a game since mid-February.
It ended in the most dramatic fashion possible as replacement fly-half Valentin Courrent converted scrum-half Jonathan Pelissie's try after the final hooter.
Toulon were cruising after tries from Georgian prop Levan Chilachava, Samoan wing David Smith and Kiwi centre Rudi Wulf had the Heineken Cup semi-finalists well in control.
RCT took the foot of the gas and an intercept try from Grenoble wing Florian Ninard had the jam-packed stadium on its feet.
The hosts struck again after Toulon prop Xavier Chiocci was sent to the sin bin and the home pack rumbled over after an attacking line-out.
With their tails up, it was all Grenoble in the finals stages and Pelissie's try on the stroke of full time set up a fairytale ending for last season's Pro D2 champions, who have enjoyed an impressive first season in the top division.
Tries: Ninard, Hegarty, Pelissie
Cons: Pelissie, Courrent
Pens: Pelissie 6
Tries: Chilachiva, Smith, Wulf
Cons: Giteau 3
Yellow card: Chilachiva, Chiocci
Grenoble: 15 Fabien Gengenbacher, 14 Lucas Dupont, 13 Aloisio Butonidualevu, 12 Nigel Hunt, 11 Florian Ninard, 10 Blair Stewart, 9 Jonathan Pelissie, 8 Shaun Sowerby, 7 Jonathan Best, 6 Roland Bernard, 5 Andrew Farley, 4 Benjamin Hand, 3 Dayna Edwards, 2 Vincent Campo castillo, 1 Albertus Buckle.
Replacements: 16 Gregory Fabro, 17 Anthony Hegarty, 18 Cyril Veyret, 19 Geoffrey Nouhaillaguet, 20 Rida Jaouher, 21 Joaquin Tuculet, 22 Valentin Courrent, 23 Feliuaki Taukeiaho.
Toulon: 15 Benjamin Lapeyre, 14 Vincent Martin, 13 Rudi Wulf, 12 Geoffroy Messina, 11 David Smith, 10 Matt Giteau, 9 Nicolas Durand, 8 Joe van Niekerk, 7 Pierrick Gunther, 6 Virgile Bruni, 5 Jocelino Suta, 4 Simon Shaw, 3 Levan Chilachava, 2 Sebastien Bruno, 1 Gethin Jenkins.
Replacements: 16 Mickael Ivaldi, 17 Xavier Chiocci, 18 Nicholas Kennedy, 19 Konstantine Mikautadze, 20 Juan martin Fernandez lobbe, 21 Delon Armitage, 22 Sebastien Tillous Borde, 23 Davit Kubriashvili.