Saracens must beat Rugby Parma by two points at Vicarage Road next week after they were beaten 19-18 in the first leg at a muddy Stadio Sergio Lanfranchi.
CHALLENGE CUP RESULTS
Connacht 56-3 Montpellier
Rugby Parma 19-18 Saracens
Narbonne 15-33 Sale Sharks
Pau 25-19 London Irish
Agen 23-15 Viadana
Brive 57-6 Cetransa
Clermont Auvergne 44-14 Borders
Matt Cairns and Thomas Castaignede both overcame the tough conditions to score second-half tries but Roland de Marigny slotted five kicks to win the match.
London Irish lost 25-19 at Pau after trailing 20-10 at half-time.
But Sale enjoyed a better night, winning 33-15 at French side Narbonne, with Jason Robinson dropping a goal.
Saracens coach Rod Kafer admitted his side could not cope with the waterlogged pitch.
"We're disappointed, but it's only half-time. We look forward to welcoming these guys back to Vicarage Road.
"This had to be the worst pitch I've ever seen in my life. We couldn't do anything on it except kick the ball. They adapted to the conditions better, but we're not too disheartened."
Jason Robinson was in impressive form for Sale Sharks, slotting in a 30m goal as the Zurich Premiership side built up an 18-point advantage ahead of next weekend's return match at Edgeley Park.
Steve Hanley, Sebastien Bruno and Andrew Sheridan all crossed over for Sale, while Charlie Hodgson kicked three conversions and three penalties.
Winger Mathieu Dourthe scored Pau's opening try and veteran scrum-half Philippe Carbonneau added another as London Irish slumped to defeat.
Prop Doug Wheatley scored the Zurich Premiership side's only try, with Barry Everett kicking 14
Irish side Connacht mauled Montpellier 56-3 at the Sportsground, with fly-half
Paul Warwick bagging 26 points in another impressive display.
Connacht ran in four tries in the first half to lead 26-3, and coasted to a
comfortable win despite coach Mike Bradley clearing his bench when the game was
in the bag.
Meanwhile, in the European Shield, Worcester thumped Rugby Rovigo 48-3 at Stadio Communale.