A 39-19 win over Bayonne on Saturday saw Harlequins finish top of Pool One in the Amlin Challenge Cup and qualify for the quarter-finals.
A brace of tries from livewire centre George Lowe proved the turning point after the visitors led 14-13 at half-time at The Stoop.
Lowe crossed twice inside seven minutes to give Quins a decisive lead, ghosting through some poor Bayonne defence just 40 seconds after the restart before showing his pace with a classy outside break with 47 minutes on the clock.
Nick Evans kicked 14 points, scored a superb individual try and made another for England and Lions wing Ugo Monye.
Evans set Quins on their way with only five minutes played but Bayonne hit back with a Ross Filipo score and three penalties from Cedric Garcia to lead by a point at the break.
Lowe then struck twice in quick succession before Monye scored with eight minutes remaining. Sam Smith added a fifth with the last play of the game.
In Pool One's other game, Connacht ran out easy 83-7 winners over Cavalieri.