England and Saracens flyhalf Owen Farrell successfully kicked 11 shots at goal to earn Saracens what had looked like an unlikely victory over Racing Metro at Stade de la Beaujoire, Nantes.
Saracens fought their way back from being thirteen points behind early in the first half to win the match by nine points.
Saracens lost winger David Strettle in the third minute of the match and after the former England winger was stretchered off Racing Metro exposed the visitors on the wing and ran in three early tries.
The 2010/11 English Premiership Champions looked like they were heading for a hammering as Juan Imhoff followed Luc Ducalcon over the tryline twice in the first half.
However Saracens managed to settle the game down and slowly turned the momentum into their favour and only trailed by seven points at 25-18 at half time through six first half Farrell penalties.
In the second half Farrell gave the French Top 14 hopefuls a master class in composure and kicking as he racked up a total of 32 points for the match.
Farrell punished Racing's indiscipline with 10 penalty goals - though Sarries were not entirely blameless and had to do it the hard way after twice being reduced to 14 men with Will Fraser and Chris Ashton spending spells in the sin bin.
Farrell kicked everything that came his way, including the conversion of full back Chris Wyles' try, after Farrell had inevitably provided the platform for the score from a line-out with his pin-point touchfinder.
The win took Steve Borthwick's team to 18 points with a regulation win at home against Edinburgh Rugby on Sunday good enough for a last eight place.
Racing are now out of the hunt on 12 points - Farrell's final last-gasp penalty denying them even a sniff of a losing bonus point - and Munster, who play Edinburgh at Murrayfield in their Round 5 game, are the only remaining threat for group honours.
But it was Farrell's masterclass with the boot that decided this make-or-break contest as Racing outscored them 3-1 in tries.
Prop Luc Ducalon got the first after just six minutes and Pumas wing ace Juan Imhoff got two in just four minutes at the end of the first quarter.
It all added up to Saracens finding themselves 13 points adrift three times before they steadied the ship and staged a fightback brimming with never-say-die character.
Final Score Racing Metro 28 (25) Saracens 37 (18)
Tries: Ducalcon, Imhoff 2
Penalties: Descons 3
Conversions: Descons 2
Penalties: Farrell 10
Referee: Alain Rolland (Ire),
Assistant ref: Mark Patton (Ire),
Assistant ref: Brian MacNeice (Ire),
TMO: Simon McDowell (Ire),
Citing Commisioner: Eddie Walsh (Ire)
Racing Metro 92: Benjamin Fall; Juan Imhoff, Henry Chavancy, Fabrice Estebanez, Sireli Bobo; Juan Martin Hernandez, Sebastien Descons; Eddy Ben Arous, Dimitri Szarzewski (c), Luc Ducalcon, Francois Carl Van der Merwe, Jone Qovu Nailiko, Antoine Battut, Alvaro Galindo, Sakiusa Matadigo.
Replacements: Benjamin Noirot, Julien Brugnaut, Benjamin Salemane Sa, Fabrice Metz, Camille Gerondeau, Jacques Cronje, Mathieu Belie, Olly Barkley.
Saracens: Chris Wyles; Chris Ashton, Joel Tomkins, Brad Barritt, David Strettle; Owen Farrell, Richard Wigglesworth; Rhys Gill, Schalk Brits, Matt Stevens, Steve Borthwick (c), Mouritz Botha, Kelly Brown, Will Fraser, Ernst Joubert.
Replacements: John Smit, Mako Vunipola, Petrus Du Plessis, George Kruis, Nic Fenton-Wells, Neil de Kock, Charlie Hodgson, Duncan Taylor.
With thanks to ERCRugby.