French side Clermont ended two-time defending European Cup champions Leinster's 17-match unbeaten run in the competition, beating the Irish province 15-12.
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 flyhalf 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.
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 scrumhalf Lloyd Williams got a deserved red card from Irish referee John Lacey for a terrible spear tackle on his opposite number Benoit Paillaugue.
Lacey, a who had scored two tries on his previous trip to Cardiff's ground for Munster before injury ended his career, had no hesitation once he had separated the two sets of players in sending off Williams.
However, the hosts shrugged off that setback and stayed in touching distance of the 2011 French rugby finalists, who scored just the one try in the remainder of the first half through Pierre Berard in the 31st minute.
Paillaugue, though, kept the French side's score ticking over but he was matched by the highly-impressive Blues flyhalf, 19-year-old Rhys Patchell, who not only scored all their points, including a drop-goal, but also produced a try saving tackle on Berard with an hour gone.
Paillaugue gave the visitors some breathing space with a penalty soon after for 22-18 and then the outstanding Georgian flank Mamuka Gorgadze looked to have sealed the game when he stretched over the line to touch down.
However, Patchell slotted over two quick penalties - the second from distance - to reduce the lead to just three points with nine minutes remaining but Paillaugue kicked a penalty of his own to make it 30-24.
With Cardiff having to throw caution to the wind and score a try of their own it was little surprise they conceded a third, a well worked one two minutes from time from Fijian Timoci Nagusa, who touched down in the corner after being set up by man of the match Gorgadze.
Clermont 15-12 Leinster
Pens: James 4
Pens: Sexton 4
Clermont: 15 Lee Byrne, 14 Sitiveni Sivivatu, 13 Aurélien Rougerie (captain), 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 (captain), 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)
TMO: Derek Bevan (Wales)
Montpellier 35-24 Cardiff Blues
For Cardiff Blues:
Pens: Patchell 7
Red card: L Williams
Tries: Bérard, Gorgodze, Nagusa
Pens: Paillaugue 6
Cardiff Blues: 15 Jason Tovey, 14 Alex Cuthbert, 13 Gavin Evans, 12 Dafydd Hewitt, 11 Harry Robinson, 10 Rhys Patchell, 9 Lloyd Williams, 8 Andries Pretorius (captain), 7 Sam Warburton, 6 Josh Navidi, 5 James Down, 4 Lou Reed, 3 Scott Andrews, 2 Marc Breeze, 1 Sam Hobbs.
Replacements: 16 Rhys Williams, 17 Tafa'ao Filise, 18 Benoit Bourrust, 19 Macauley Cook, 20 Robin Copeland, 21 Lewis Jones, 22 Ceri Sweeney, 23 Owen Williams.
Montpellier: 15 Benjamin Thiery, 14 Yoan Audrin, 13 Thomas Combezou, 12 Paul Bosch, 11 Pierre Bérard, 10 Santiago Fernandez, 9 Benoit Paillaugue, 8 Johnnie Beattie, 7 Mamuka Gorgodze, 6 Alexandre Bias, 5 Aliki Fakate, 4 Drickus Hancke, 3 Maximiliano Bustos, 2 Agustin Creevy, 1 Mikheil Nariashvili.
Replacements: 16 Rassie van Vuuren, 17 Yvan Watremez, 18 Barry Fa'amausili, 19 Mickael Demarco, 20 Kelian Galletier, 21 Eric Escande, 22 Shontayne Hape, 23 Timoci Nagusa.
Referee: John Lacey (Ireland)
Assistant referees: Leo Colgan (Ireland), Michael Black (Ireland)
TMO: Peter Ferguson (Ireland)
Leicester Tigers 33-25 Treviso
Tries: Penalty Try, Tuilagi, Salvi 2
Cons: Ford 4
Tries: Budd, Cittadini, Loamanu
Cons: Di Bernardo 2
Pens: Di Bernardo 2
Leicester: 15 Mathew Tait, 14 Scott Hamilton, 13 Manusamoa Tuilagi, 12 Dan Bowden, 11 Vereniki Goneva, 10 George Ford, 9 Ben Youngs, 8 Thomas Waldrom, 7 Julian Salvi, 6 Ed Slater, 5 Geoff Parling (captain), 4 Graham Kitchener, 3 Martin Castrogiovanni, 2 Tom Youngs, 1 Marcos Ayerza.
Replacements: 16 Rob Hawkins, 17 Logovi'i Mulipola, 18 Dan Cole, 19 Jordan Crane, 20 Richard Thorpe, 21 Micky Young, 22 Geordan Murphy, 23 Matt Smith.
Treviso: 15 Luke McLean, 14 Christian Loamanu, 13 Tommaso Benvenuti, 12 Luca Morisi, 11 Ludovico Nitoglia, 10 James Ambrosini, 9 Edoardo Gori, 8 Manoa Vosawai, 7 Dean Budd, 6 Simone Favaro, 5 Valerio Bernabò, 4 Antonio Pavanello (captain), 3 Ignacio Fernandez Rouyet 2 Franco Sbaraglini, 1 Alberto De Marchi.
Replacements: 16 Leonardo Ghiraldini, 17 Michele Rizzo, 18 Lorenzo Cittadini, 19 Cornelius Van Zyl, 20 Francesco Minto, 21 Paul Derbyshire, 22 Fabio Semenzato, 23 Alberto Di Bernardo.
Referee: Pascal Gauzere (France)
Assistant referees: Laurent Cardona (France), Stephan Pomarède (France)
TMO: Gilles Cogne (France)