RONAN O’Gara’s boot took Munster to a 15-9 win over Saracens in the other Pool 1 clash at Thomond Park.
O’Gara kicked five out of five while his young counterpart Owen Farrell missed four of his seven efforts in a game of no tries.
Farrell redeemed himself by converting a 79th-minute penalty for a losing bonus point which keeps Saracens at the top of the pool with Munster now level on ten points ahead of next week’s return fixture in Watford.
Munster edged the first half and led 9-3 at the interval.
O’Gara hammered over two more kicks to stretch the lead to 12 points, although Farrell had the final say by coolly completing his penalty hat-trick in the dying minutes.
The immense physicality of the game spilled over at times, leading to yellow cards for Donncha O’Callaghan and Rhys Gills.
Munster head coach Rob Penney believes the return clash is vital. He said: “We are in a tough pool but Sunday’s game is a must-win.”
Mark McCall, Saracens’ director of rugby, said: “We were well below our best. It was a disappointing performance. It is not often in sport that you get a chance to rectify what’s gone wrong a week later against the same opposition. That is the beauty of these back-to-back matches.”