Brive will have to come from behind for the third time this season in the Euroepan Challenge Cup if they are to reach their third major European final.
The 1997 Heineken Cup winners were kicked to defeat by Pau's France A outside half Ludovic Mercier at Stade Hameau as he helped himself to 22 points with the boot and also scored a try.
Mercier's 27 point haul proved the major difference between the two sides and means Pau are well on course to reach their second Challenge Cup final. In 2000 they took the title when they beat Castres 34-21 in Toulouse.
Having led by nine points at half-time, the home side weathered an early storm from Brive's back line and notched 15 unanswered points in the final quarter to give themselves a 21 point cushion going into the second leg in Brive in three weeks time.
Brive are no strangers in having to overcome a first leg defeat and they recovered at their own ground to beat Worcester in the first round, turning a 14 point deficit into a 16 point victory, and Saracens in the quarter-finals, when they turned a three point defeat into a 12 point victory.
Mercier gave Pau the perfect start with two penalties in the opening seven minutes and by half-time he was in the groove having kicked four penalties and converted Mathieu Dourthe's try two minutes before the break.
Brive conjured up a try for their French wing Julien Laharrague which scrum half Vincent Courrent converted to give them the lead briefly, but after that they could only manage three more Courrent penalties.
Two of those kicks came either side of the interval to cut the home lead to six points after 51 minutes, but Mercier steadied the ship with his fifth penalty, a drop goal and a try.
Nicolas Martin ran in a third Pau try on the stroke of time to complete a miserable afternoon for Brive, who now face an uphill task in the second leg.