Leinster picked up a losing bonus point in a titanic clash with Pool Five leaders Clermont Auvergne at the Stade Marcel Michelin.
Despite a first defeat in this season's Heineken Cup, the defending champions emerged with a clutch of positives ahead of next Saturday's return date in Dublin.
Sean O'Brien had an impressive return to European action in a barnstorming back row unit, with Fergus McFadden and cup debutant Andrew Goodman the pick of the visitors' backs.
Jonathan Sexton kicked four penalties for the province, a tally matched by Clermont's Morgan Parra, with Brock James landing a crucial drop goal just before half-time.
There was a raucous atmosphere in Clermont's home fortress, where the French club boast an unbeaten run that recently passed 50 games.
Clermont drew first blood, Parra popping over a penalty from in front of the posts but Leinster carried more of an early threat with ball in hand.
Jamie Heaslip's inside pass put Ian Madigan through a midfield hole and Sexton stepped up to convert the subsequent penalty.
An obstruction from a lineout went unpunished as Sexton missed a tricky penalty, with play swinging back and forth at a hectic pace.
A strong burst from Napolioni Nalaga led to Parra kicking Clermont back in front by the end of a defence-dominated first quarter.
The home side cranked it up as Sitiveni Sivivatu drew in two defenders, releasing Aurelien Rougerie and Lee Byrne for a right-wing surge that almost led to a try.
A third successful penalty from Parra stretched the margin to 9-3, but Leinster were quick to respond through a bout of sturdy carries from McFadden, Cian Healy and Kevin McLaughlin.
Sexton was back on target with a penalty from the 22 and as the Leinster pack exerted more control, they continued to make good ground.
Madigan burst into the 22 and with Clermont penalised for not rolling away, Sexton brought the title holders level.
However, Clermont hit back through another penalty from Parra and James' late drop goal put six points between the sides at the break.
Sexton and Madigan probed with clever kicks on the resumption, while Isa Nacewa and McFadden both went close to collecting cross-field kicks from the Leinster number 10. Wales and Lions full-back Byrne did brilliantly to deny the former.
After two confidence-boosting scrums, with Mike Ross shunting Vincent Debaty backwards, Sexton slotted his fourth successful penalty with 54 minutes on the clock.
Clermont looked to be tiring but Byrne did well again to sweep the ball into touch with Nacewa bearing down on him as he chased Gordon D'Arcy's kick.
Although the French outfit responded with two side-stepping runs from the dangerous Wesley Fofana, Leinster defended stoutly but a couple of loose lineouts - with replacement hooker Richard Strauss on the pitch - robbed them of promising attacking positions as time ran down.
The result leaves Clermont with a five-point buffer at the top of the table. The sides will regroup for a vital second meeting at the Aviva Stadium next Saturday afternoon.