Racing-Mètro effectively ended Saracens' hopes of making the Heineken Cup quarter-finals with a 24-21 win in Watford on Saturday.
Racing are the first French team ever to win at Vicarage Road as a late rally from the English hosts proved too little too late.
Juan Martin Hernandez and Sereli Bobo touched down on either side of half time while Francois Steyn kicked two penalties from his own half to hand the French club a deserved victory.
Noah Cato scored in the early minutes and Bradley Barritt touched down at the death for Sarries, but the home side struggled to trouble to Racing defence for most of the game.
The result leaves Saracens without a win in their three starts while Racing are right back in the running to get out of Pool Two ahead of Leinster's trip to Clermont on Sunday.
Sarries got off to a perfect start when Cato touched down in the corner after just five minutes, beating two defenders to end a move that started 50m up field.
Fly-half Owen Farrell missed the conversion but found the target soon afterwards to give the hosts an 8-0 lead.
Racing clawed their way back into contention with a penalty from Jonathan Wisniewski and a 55m effort from Steyn before taking the lead when Hernandez touched down.
The Argentine full-back was gifted his try when Barritt inexplicably flicked the ball backwards into play while under pressure in the corner leaving Hernandez with nothing more to do but flop onto the loose ball.
Wisniewski's conversion gave the visitors a 13-8 lead which they took to half-time.
The Parisians moved further ahead after the break when Steyn nailed his second penalty from his own half. But Sarries were quick to reply as Farrell reduced the gap to five points from the kicking tee entering the final quarter.
Sebastien Chabal was literally foaming at the mouth when he was adjudged to have left his feet and Farrell narrowed the difference to just two points.
Saracens failed to conserve their possession however allowing Wisniewski to snap an opportunistic drop goal.
The game looked done and dusted with ten minutes left when Bobo sold the Sarries defence and dummy before bursting down the touchline to score.
Barritt made amends for his first-half error by barging his way over to score in the dying minutes to set up a grandstand finish.
It was all too late however as Racing now move into provisional second place in Pool Two with their three remaining games all in France.
Man of the match: It was a tough choice but we'll go with Racing hard man Sebastien Chabal who carried the ball with real venom, in the first half especially.
Tries: Cato, Barritt,
Pens: Farrell 3
Tries: Hernandez, Bobo
Saracens: 15 Chris Wyles, 14 Noah Cato, 13 David Strettle, 12 Bradley Barritt, 11 James Short, 10 Owen Farrell, 9 Neil de Kock, 8 Ernst Joubert, 7 Andy Saull, 6 Jacques Burger, 5 Mouritz Botha, 4 Steve Borthwick, 3 Petrus Du Plessis, 2 Schalk Brits, 1 Rhys Gill.
Replacements: 16 Jamie George, 17 Deon Carstens, 18 Carlos Nieto, 19 George Kruis, 20 Kelly Brown, 21 Richard Wigglesworth, 22 Nils Mordt, 23 Luke Baldwin.
Racing-Metro: 15 Juan Martin Hernandez, 14 Sereli Bobo, 13 Henry Chavancy, 12 François Steyn, 11 Mirco Bergamasco, 10 Jonathan Wisniewski , 9 Mathieu Loree, 8 Sebastien Chabal, 7 Jacques Cronje, 6 John Leo'o 5 Jone Qovu Nailiko 4 Santiago Dellape 3 Juan Pablo Orlandi 2 Carlo Festuccia 1 Andrea Lo Cicero
Replacements:16 Benjamin Noirot 17 Julien Brugnaut, 18 Johannes Coetzee, 19 Karim Ghezal, 20 Nicolas Durand, 21 Dan Scarbrough, 22 Remi Vaquiin 23 Bernard Leroux.
Venue: Vicarage Road, Watford
Referee: Alain Rolland