Harlequins could not stop Clermont Auvergne from setting up an all-French European Challenge Cup final with La Rochelle as they were beaten 32-27 at the Stade Marcel Michelin.
Paul Gustard's side produced a fine display and did little wrong, but still turned around 18 points down with first-half Clermont tries coming from Fritz Lee and Damian Penaud, with Camille Lopez kicking two drop-goals.
Mike Brown crossed for Harlequins, but Clermont's power game proved too much as Morgan Parra also added two conversions and two penalties.
After the break Harlequins went for broke and added three more tries through Chris Robshaw, James Lang and a late Alex Dombrandt effort.
Ultimately it was not enough, but the English side can take real heart from their performance ahead of their final three games of the Gallagher Premiership.
Sale Sharks meanwhile fell short in their bid to reach the decider as they were beaten 24-20 by La Rochelle in France.
A penalty try and a Chris Ashton effort meant Sale were well in the contest following an eventful first half, in which the hosts also grabbed a penalty try while Kini Murimurivalu added a second French effort.
Both teams were reduced to 14 men in the opening 40 minutes, but Gregory Alldritt's try for La Rochelle early in the second half proved crucial.
All Sale could manage in the second half was a penalty from Alan MacGinty.