Toulon overturned an 11-point half-time deficit before securing a hard-fought 20-16 victory over Leinster in Dublin on Saturday.
Anthony Etrillard's try in the 65th minute sealed Toulon's triumph after the home side raced in to a 16-5 lead at the interval.
Leinster, who are yet to win a match in the competition, delivered their best showing so far this year but sadly, for their cheering supporters, they ran out of steam in the second half.
Johnny Sexton opened up proceedings with a straight forward penalty 10 minutes in, taking advantage of a foul at the ruck as Leinster struck first blood in this all important European tie.
And in a poor couple of minutes for Toulon, they went down to 14 men temporarily. Duane Vermuelen was sent to the bin after he was caught using his hands at the ruck, as he tried to defend a try which was almost inevitable.
And his departure had little bearing, because Leinster did get the first try of the evening, a penalty one. Five minutes after his departure, the referee Wayne Barnes awarded the try. After driving from a line-out, a number of Toulon players purposely dragged it to the floor, giving Barnes the straight forward decision of handing the hosts five points.
Sexton's conversion took Leinster up into double figures. But despite the early Leinster dominance, they couldn't form a bigger lead, and the might of Toulon hit back.
Juan Smith opened their account for the night in the form of a try, receiving the ball from Florian Fresia before strolling over with relative ease.
But despite that momentary swing, Leinster kept the scoreboard ticking over. Sexton added three more after Leinster had pinned Toulon into their own 22, and added a further three on the brink of half-time to build up an 11-point gap at the break.
The second half saw a whole new Toulon side emerge, far more competitive than the first half. They should have dragged themselves back into it when Tom Taylor fed it to Drew Mitchell, but he made a mess of his handling and knocked on with a try a sure thing.
But a minute later, things were put right. The Toulon pack cranked up the pressure from a scrum metres from the Leinster try-line. And after two resets, Wayne Barnes made the straight forward decision again to equal the penalty try count. Tom Taylor's conversion cut the gap to just four with 20 left to play.
And the comeback was completed via another maul which forced its way over the line. Etrillard was the recipient of the push forwards, and spotted a small gap to nip into and put Toulon into the lead. Taylor missed out on the conversion, with just a point separating the two.
It was the turn of Toulon to make it a four point gap again, this time in their favour, with Taylor firing through three more points with five left to play.
Leinster's task was made harder with three minutes left as Jordi Murphy was shown the second yellow of the game, Leinster now needing to turn the game around a man light.
Toulon looked like sealing it with a minute to play when Mamuka Gorgodze stormed over, though after the TMO was called for it was ruled out for a knock on in the build up. Not that it mattered, as Toulon sealed a scintillating 20-16 victory.
Tries: Penalty try
Pens: Sexton 3
Yellow card: Murphy
Tries: Smith, Penalty try, Etrillard
Yellow card: Vermeulen
Leinster: 15 Rob Kearney, 14 Dave Kearney, 13 Ben Te'o, 12 Luke Fitzgerald, 11 Isa Nacewa, 10 Johnny Sexton, 9 Eoin Reddan, 8 Jamie Heaslip 7 Josh Van Der Flier, 6 Rhys Ruddock, 5 Mike McCarthy, 4 Devin Toner, 3 Mike Ross, 2 Richardt Strauss, 1 Jack McGrath
Replacements: 16 Sean Cronin, 17 Cian Healy, 18 Martin Moore, 19 Tom Denton, 20 Jordi Murphy, 21 Nick Mc Carthy, 22 Ian Madigan, 23 Zane Kirchner
Toulon: 15 Delon Armitage, 14 Bryan Habana, 13 Mathieu Bastareaud, 12 Ma'a Nonu, 11 Drew Mitchell, 10 Matt Giteau, 9 Eric Escande, 8 Duane Vermeulen 7 Steffon Armitage, 6 Juan Smith, 5 Romain Taofifenua, 4 Jocelino Suta, 3 Matt Stevens, 2 Guilhem Guirado, 1 Florian Fresia
Replacements: 16 Anthony Etrillard, 17 Xavier Chiocci, 18 Levan Chilachava, 19 Mamuka Gorgodze, 20 Juan Fernandez Lobbe, 21 Tom Taylor, 22 Sebastien Tillous-Borde, 23 Konstantine Mikautadze
Referee: Wayne Barnes (England)