Racing Métro exacted sweat revenge for two defeats last season with a 19-9 Heineken Cup victory over Edinburgh at Stade Yves du Manoir on Saturday.
The result - combined with Munster's win over Saracens - leaves Pool One wide open.
Tries from Juan Jose Imhoff and Eddy Ben Arous helped Racing inflict a third European defeat on Edinburgh this season, leaving the Scottish side without a single point in the pool.
Imhoff touched down to go alongside six points from the boot of Olly Barkley as the French outfit led 11-6 at the break, with Greig Laidlaw kicking two penalties for the visitors.
And Eddy Ben Arous' try and Maxime Machenaud's penalty in the second half confirmed the win for the Parisians, despite Laidlaw's third three-pointer.
The Gunners went into the game having failed to score a single point in two European encounters, but they broke that duck after two minutes when Laidlaw slotted a penalty.
Edinburgh's pack were in the ascendancy but Laidlaw twice went wide left in quick succession with penalty chances.
Barkley quickly put Racing on level terms with one of his own as they gained control of proceedings.
Having secured what many considered a surprising victory over Munster in Round One, the Top 14 outfit were searching for their second Heineken Cup triumph of the season. They went over for the first try of the game when excellent work from the backline ended in Guillaume Bousses feeding Imhoff, who touched down.
Their English fly-half failed to convert, but the former Bath player managed to add a second three-point opportunity.
However, Laidlaw reduced the arrears at the interval with another successful kick from the tee.
dinburgh had several chances in the second half to take the lead but they couldn't get over the try-line.
Instead, it was the hosts who proved to be clinical when Ben Arous touched down after a powerful driving maul.
Laidlaw did take the Pro12 team to within losing bonus-point territory at one stage, but Machenaud's late penalty eradicated that.
For Racing Metro:
Tries: Imhoff, Ben Arous
Pens: Barkley 2, Machenaud
Pens: Laidlaw 3
Edinburgh: 15 Greig Tonks, 14 Tom Brown, 13 Nick De Luca, 12 Matt Scott, 11 Tim Visser, 10 Greig Laidlaw, 9 Richie Rees, 8 Stuart McInally, 7 Roddy Grant, 6 David Denton, 5 Sean Cox, 4 Grant Gilchrist, 3 Geoff Cross, 2 Ross Ford, 1 John Yapp.
Replacements: 16 Andy Titterrell, 17 Allan Jacobsen, 18 WP Nel, 19 Perry Parker, 20 Rob McAlpine, 21 Piers Francis, 22 Douglas Fife, 23 Lee Jones.
Racing Metro: 15 Juan Martin Hernandez, 14 Virimi Vakatawa, 13 Guilliaume Bousses, 12 Fabrice Estebanez, 11 Juan Imhoff, 10 Olly Barkley, 9 Sebastien Descons, 8 Masi Matadigo, 7 Jacques Cronje, 6 Antoine Battut, 5 Francois Van der Merwe, 4 Fabrice Metz, 3 Benjamin Salemane Sa, 2 Thomas Bianchin, 1 Julien Brugnaut.
Replacements:16 Dimitri Szarzewski 17 Eddy Ben Arous 18 Juan Pablo Orlandi 19 Bernard Le Roux 20 Jone Qovu Nailiko 21 Camille Gerondeau 22 Maxime Machenaud 23 Alexandre Dumoulin
Referee: Peter Fitzgibbon (Ireland)