The visitors had looked to be cruising to the top of the pool when they secure a bonus-point by half-time thanks to an early try from Xavier Mignot and a fine hat-trick from Sona Taumalolo, with Gilles Bosch adding all four conversions.

Agen only registered a Bautista Guemes penalty during the opening half, but they clawed their way back into the game as Marc Baget, Marc Barthomeuf and Marius Deguilhem all scored tries, but a late penalty try and Mignot’s second score sealed the win for Grenoble.

Grenoble had scored a last-minute try to win their previous pool match against London Irish, and they kicked off this encounter with a try inside the opening 60 seconds as Mignot raced over with Bosch converting. Within three minutes prop Taumalolo was crashing his way over the line from close range to further extend the advantage.

Guemes at least stemmed the tide with a penalty, but when the Grenoble pack again asserted their dominance, Taumalolo ploughed over the line again and, remarkably, followed up with another try just five minutes later to round off a 34 minute hat-trick for the loosehead prop.

With Bosch adding the extras, the visitors were 25 points to the good at the interval and there appeared no way back for Agen. However, the hosts flew out of the blocks at the start of the second half and a period of extended pressure ended with number eight Baget working his way over.

Replacement hooker Barthomeuf rounded off a fine flowing move to narrow the gap further, with Grenoble’s Jonathan Best seeing yellow for infringing in the build-up, to give Agen further hope. Francois Bouvier converted, and when wing Deguilhem was sent scuttling over after sharp midfield play an incredible comeback win looked on the cards.

But when a powerful Grenoble scrum put the home side in reverse and forced a penalty try, plus a yellow card for hooker Barthomeuf, the balance shifted back in Grenoble’s favour, with Mignot crossing for his second to put the result beyond doubt.