Saracens took control of Heineken Cup Pool One after beating visitors Munster in an absorbing clash at Vicarage Road.
Saracens fly-half Owen Farrell and Munster counterpart Ronan O'Gara exchanged penalties before David Strettle's try put the hosts ahead.
Munster captain Doug Howlett touched down after James Downey's interception, but Farrell's three second-half kicks put Saracens top with two games left.
O'Gara missed a penalty for 16-16 as Munster claimed a losing bonus point.
Both sides started the fourth round of matches locked on 10 points and knew victory would allow them to leapfrog Racing Metro, who moved top by beating Edinburgh on Friday, at the top of Pool One.
Questions had been raised about Farrell's temperament after he missed four of his seven penalty attempts as Saracens slumped to a 15-9 defeat in the reverse fixture last weekend.
But Sarries boss Mark McCall decided to switch Farrell to number 10 at the expense of Charlie Hodgson - and his faith was justified as the England international kept his cool to kick his team to victory.
Qualification is now firmly in the hands of the Premiership side who would qualify for the knockout stage with wins against Racing Metro and Edinburgh.
However two-time champions Munster, who have only failed to reach the quarter-finals once in the past 14 seasons, remain firmly in the hunt for a place as one of the two best-placed runners up.
Munster looked nervy in the opening 10 minutes as Farrell pushed an early drop-goal attempt wide before landing a long-range penalty.
O'Gara, the all-time leading points scorer in the Heineken Cup, missed his first opportunity but immediately made amends after pressure from the Munster pack forced Farrell into holding on to a high ball.
With qualification stakes high, there was a raucous atmosphere around Vicarage Road from kick-off. But the noise levels were raised even higher when Strettle raced on to a beautifully-weighted chip from Richard Wigglesworth that sliced open a flat Munster defence, to touch down.
However, the home side's lead lasted just five minutes before Munster were presented the simplest of opportunities to cancel out Strettle's try. Downey telegraphed Farrell's pass to intercept and race clear before offloading to the supporting Howlett, who provided an extra burst of energy to surge over the line.
With the pouring rain starting to become more influential, the home side resumed in the same vein at the start of the second half as Munster continued to soak up the pressure.
O'Gara's penalty was cancelled out by two more from Farrell before Saracens were reduced to 14 men when flanker Will Fraser was penalised for a high tackle.
Munster tried to press their numerical advantage as they pounded the Saracens defence, but the hosts remained firm and forced O'Gara into a drop goal attempt, which he put wide.
Uncharacteristically, the Ireland international pushed wide another in-range penalty and Farrell's fourth effort sealed a potentially vital win.
Saracens director of rugby Mark McCall:
"Hopefully this was a bit of a coming of age for us, but after four games you want to control your destiny.
"We have to go to Racing Metro and potentially there are more twists and turns to come in this group.
"We played some really good stuff in attack and dug in when we needed to. Our performance had a bit of everything and we showed real character."
Munster boss Rob Penney:
"We have still 10 points to play for and who knows what will happen.
"I am very optimistic that this group of players can turn this around as they are hugely talented.
"I can't praise the boys highly enough, but their scrum improved substantially and their lineout was very good."
Saracens: Alex Goode; Chris Ashton, Joel Tomkins, Brad Barritt, David Strettle; Owen Farrell, Richard Wigglesworth; Rhys Gill, Schalk Brits, Matt Stevens; Steve Borthwick (capt), Mouritz Botha; George Kruis, Will Fraser, Kelly Brown.
Replacements: Wyles for Goode (62), De Kock for Wigglesworth (51), Vunipola for Gill (41), Smit for Brits (60). Not Used: Du Plessis, Sheriff, Joubert, Hodgson.
Sin Bin: Fraser (62).
Munster: Felix Jones; Doug Howlett (capt), Casey Laulala, James Downey, Simon Zebo; Ronan O'Gara, Conor Murray; Dave Kilcoyne, Mike Sherry, BJ Botha; Donncha O'Callaghan, Donnacha Ryan; Dave O'Callaghan, Peter O'Mahony, James Coughlan.
Replacements: O'Dea for Jones (18), Horan for du Preez (71), Archer for Botha (78), Butler for P. O'Mahony (67). Not Used: Varley, B. Holland, Williams, Keatley.
Ref: Jerome Garces (France).