Saracens (8) 21
Tries: Cato, Barritt Cons: Farrell Pens: Farrell 3
Racing Metro (13) 24
Tries: Hernandez, Bobo Cons: Wisniewski Pens: Wisniewski, Steyn 2 Drop-goals Wisniewski
Bobo's try gave Racing a decisive lead at Vicarage Road on Saturday
Saracens' Heineken Cup hopes are over for the season after Racing Metro survived a dramatic late fightback to secure victory at Vicarage Road.
The hosts took an early 8-0 lead via a Noah Cato try and Owen Farrell penalty.
But Racing hit back with three penalties and a converted try for Argentine Juan Martin Hernandez.
Farrell landed two more penalties but a Jonathan Wisniewski drop-goal and Sireli Bobo try saw Racing home despite Brad Barritt's converted late try.
Victory means the Parisian side will relish next Friday's return fixture in the French capital, where they also entertain fellow contenders Leinster in the final round of pool fixtures in January.
Saracens, who only have two points after a third straight pool defeat, only have qualification for the second-tier Amlin Challenge Cup to aim for, but even that is a long shot.
When the hosts started brightly it looked as though they might be on their way to securing their first win in Europe this season.
But although they took the lead with a superb try, a series of mistakes and poor decision-making ensured Racing are now in good shape to mount a challenge to Leinster and Clermont Auvergne.
The hosts took an early lead as livewire hooker Schalk Brits sparked a long-range counter attack.
After the ball went through numerous pairs of hands, David Strettle, playing at outside centre, gave Cato the chance to run at the Racing defence.
The young winger stepped out of the tackle of South Africa centre Francois Steyn and powered through the covering defender to score.
Farrell could not land the conversion but was on target with a penalty, only for Wisniewski to reply in kind within a couple of minutes.
Steyn, renowned for the power of his boot, thumped over a long-range penalty just before the 20-minute mark to make it 8-6 and the visitors were then handed a try on a plate.
Bobo's chip to the corner put the Sarries defence under pressure but Barritt's desperate pass back inside was suicidal.
And Hernandez - fighting his way back after injury and still a long way short of the form that lit up the 2007 World Cup - was able to simply fall on the ball and score, with Wisniewski's conversion putting the visitors 13-8 ahead at half-time.
Farrell spurned the chance to trim the gap when he missed a kickable penalty after 51 minutes.
Steyn then banged over a long-range effort three minutes later, but Farrell kept the hosts in touch with two penalties of his own.
The 19-year-old fly-half was then charged down to set up the platform for Wisniewski to pop over a vital drop-goal, before Bobo made the game safe with a stunning score.
He sparked a surging counter-attack from his own 22 and when Racing attacked down their right again, the powerful Fijian flyer beat Jacques Burger and Chris Wyles as he stormed his way to the line.
Wisniewski converted to make the game safe, although Saracens refused to lay down and die and instead rallied to secure a losing bonus point.
Strettle ruined the chance of one try as he failed to pass inside with only the full-back to beat but the hosts came again and Barritt crossed for a converted score.
The hosts mounted one desperate last attack but lost control of the ball as Racing confirmed they are genuine contenders to progress from Pool Two.
Saracens head coach Mark McCall:
"We're very disappointed after three games in our pool which have all been very close, all of which we could have won yet we finished up losing all three.
"But we won't pack things in. We'll go out to win all of our remaining games with the belief that we can still have an impact on this pool. "We gave away some poor penalties and there was a small mistake that led to a try but you must remember they're a very good rugby team.
"Right at the end we had them on the ropes but that's when their experience showed. It's a learning curve for a lot of our lads."
Racing Metro coach Simon Mannix:
"The measure of a team is how they defend and I thought we defended extremely well when we had to.
"We came here knowing Saracens are a quality side who would give us a hell of a fight.
"Our pool is probably the toughest given the four sides in it and it could still go down to the wire.
"I see this win as a turning point for us in a very, very difficult competition."
Saracens: Wyles; Cato, Strettle, Barritt, Short; Farrell, De Kock, Gill, Brits, Du Plessis, Borthwick, Botha, Burger, Saull, Joubert.
Replacements: Mordt for Cato (71), Wigglesworth for de Kock (52), Carstens for Gill (52), George for Brits (52), Nieto for Du Plessis (52), Kruis for Botha (52), K. Brown for Joubert (71).
Not Used: Baldwin.
Racing Metro 92: Hernandez; Bobo, Chavancy, Steyn, M Bergamasco; Wisniewski, Loree; Lo Cicero, Festuccia, Orlandi, Dellape, Nailiko, Leo'o, Cronje, Chabal.
Replacements: Scarbrough for Hernandez (65), Noirot for Festuccia (65), Ghezal for Dellape (71), Vaquiin for Leo'o (53), Le Roux for Cronje (68).
Not Used: Brugnaut, Coetzee, Durand.
Ref: Alain Rolland (Ireland).