Leinster's 17-match unbeaten run in the Heineken Cup came to an end at Marcel Michelin as Clermont edged to a 15-12 Pool victory on Sunday.
The French side's success means they now stretch their home record to 51 games and also extend their advantage in Pool 5 to five points.
Leinster now must regroup before les Jaunards arrive in Dublin for the return fixture next Saturday, with a home win now imperative for the holders.
There were no tries in the game while only three points were scored in the second-half as it had the feel of an arm-wrestle between two giants.
In the end it was a smart drop from Brock James on 39 minutes that proved the difference as both Jonathan Sexton and Morgan Parra struck four penalties apiece in the closely-fought contest.
Parra gave Clermont a 9-6 lead by the half-hour mark with three shots at goal while his Leinster counterpart Sexton - playing his 100th competitive game for the Irish province - knocked over two.
Ireland international fly-half Sexton pulled his side level in the 33rd minute as Leinster dominated the game in an fine spell of patient rugby without being able to turn it into tries.
Referee Nigel Owens was then forced to give the two captains a warning that he would be forced to bin players soon with the number of infringements taking place in the teams 22.
It was from a resulting offence that Parra restored the hosts lead with another penalty for 12-9 and fly-half James dropped a goal on the stroke of half-time to give Clermont a 15-9 lead.
Sexton reduced the deficit in the 53rd minute as Leinster dominated again, but still came up short in crossing the line - replacement hooker Richardt Strauss not helping with three terrible throws at line-outs close to the Clermont line.
That was the only score in the half as the match somewhat failed to live up to its billing.
Pen: James 4
Pen: Sexton 4
Clermont: 15 Lee Byrne, 14 Sitiveni Sivivatu, 13 AurÃ©lien Rougerie (c), 12 Wesley Fofana, 11 Naipolioni Nalaga, 10 Brock James, 9 Morgan Parra, 8 Damien Chouly, 7 Alexandre Lapandry, 6 Julien Bonnaire, 5 Nathan Hines, 4 Jamie Cudmore, 3 Davit Zirakashvili, 2 Benjamin Kayser, 1 Vincent Debaty.
Replacements: 16 Ti'i Paulo, 17 Raphael Chaume, 18 Daniel Kotze, 19 Julien Pierre, 20 Julien Bardy, 21 Ludo Radosavljevic, 22 David Skrela, 23 Regan King.
Leinster: 15 Ian Madigan, 14 Fergus McFadden, 13 Gordon D'Arcy, 12 Andrew Goodman, 11 Isa Nacewa, 10 Jonathan Sexton, 9 Issac Boss, 8 Jamie Heaslip, 7 Sean O'Brien, 6 Kevin McLaughlin, 5 Damien Browne, 4 Leo Cullen (capt), 3 Mike Ross, 2 Sean Cronin, 1 Cian Healy.
Replacements: 16 Richardt Strauss, 17 Jack McGrath, 18 Michael Bent, 19 Devin Toner, 20 Shane Jennings, 21 Eoin Reddan, 22 Fionn Carr, 23 Andrew Conway.
Referee: Nigel Owens (Wales)
Assistant referees: Sean Brickell (Wales), Jonathan Mason (Wales)
Television match official: Derek Bevan (Wales)
Timekeeper: Daniel Gillet (France)