Paris - French side Clermont ended two-time defending European Cup champions Leinster's 17-match unbeaten run in the competition on Sunday beating the Irish province 15-12 in a gripping encounter.
Clermont's win - their 51st successive home win and who were the last side to beat Leinster in 2010 - puts them five points clear of the Irish side ahead of the return fixture in Dublin next weekend.
Morgan Parra gave Clermont a 9-6 lead by the half hour mark with three penalties while his Leinster counterpart Jonathan Sexton - playing his 100th competitive game for the Irish province - converted two.
Sexton pulled them level in the 33rd minute as Leinster dominated the game without being able to turn it into tries.
Referee Nigel Owens was forced to give the two captains Leo Cullen and Aurelien Rougerie a warning that he would be forced to sin bin someone soon with the number of infringements taking place in the teams' 22.
It was from the latest offence that Parra restored the hosts lead with another penalty for 12-9 and Australian fly-half Brock 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.
Clermont coach Vern Cotter said it had been an engrossing physical contest.
"It was a good old boxing battle out there from the start," said the New Zealander.
"We got what we wanted which was the four points and I'm happy with the win if not with the second-half performance.
"Hopefully this will raise our confidence levels and we won't be as badly beaten as we were in Dublin two years ago."
His Leinster counterpart Joe Schmidt, who was assistant to Cotter at Clermont from 2007-2010, took defeat graciously.
"It's incredibly disappointing, we dominated the game but lost three crucial lineouts," said the Kiwi.
"I couldn't have asked for more from the players. Hopefully we will get some of our injured players back and salvage one of the qualifying spots."
Earlier French side Montpellier ran out 35-24 winners over Cardiff Blues in their pool match but they made hard work of winning it playing against 14 men for 55 minutes of the match.
Victory kept Montpellier, coached by former France captain Fabien Galthie, in with a chance of reaching the knockout stage, though, they trail Top 14 rivals Toulon by five points with three games remaining.
Defeat for Cardiff left them on just one point and continued the miserable run of Welsh clubs in this season's competition with only one win in nine matches for the three sides involved.
Cardiff put up a spirited display despite being reduced to 14 men in the 25th minute as scrum-half Lloyd Williams got a deserved red card from Irish referee John Lacey for a terrible spear tackle on his opposite number Benoit Paillaugue.
However, the hosts shrugged off that setback and stayed in touching distance of the 2011 French rugby finalists till the very end - their 19-year-old fly-half Rhys Patchell putting in a superb performance with all their points.
The visitors put a unfair hue on the final score with their third try in the dying seconds, Timoci Nagusa touching down in the corner after he was set up by man of the match Mamuka Gorgadze.
European Cup results on Sunday:
Leicester (ENG) 33 Treviso (ITA) 25
Clermont (FRA) 15 Leinster (IRL) 12
Cardiff Blues (WAL) 24 Montpellier (FRA) 35
Played on Saturday:
Munster (IRL) 15 Saracens (ENG) 9
Racing Metro (FRA) 19 Edinburgh (SCO) 9
Toulouse (FRA) 30 Ospreys (WAL) 14
Zebre (ITA) 14 Harlequins (ENG) 57
Scarlets (WAL) 16 Exeter (ENG) 22
Sale (ENG) 6 Toulon (FRA) 17
Played on Friday:
Connacht (IRL) 22 Biarritz (FRA) 14
Glasgow (SCO) 6 Castres (FRA) 9
Northampton (ENG) 6 Ulster (IRL) 25