Tom Williams grabbed two tries as a young Harlequins side beat Bath 31-23 in the LV= Cup semi-finals at The Stoop on Saturday.
Tom Guest and Karl Dickson also touched down with fly-half Ben Botica kicking 11 points, converting all four tries before putting the issue beyond doubt with a late penalty.
Bath replied with tries from lock Francis Louw and full-back Nick Abendanon, with fly-half Tom Heathcote kicking their other 13 points.
Quins will play the winners of tomorrow's semi-final between Sale Sharks and Saracens in the final at Sixways, Worcester, on Sunday, March 17.
Harlequins struck the first blow in the eighth minute when number eight Guest finished a superb move off the back of a lineout.
Scrum-half Dickson found Botica, his inside pass released winger Williams and flanker Maurie Fa'asavalu sent Guest on a clear run to the posts to leave Botica an easy conversion.
Bath, prompted by 35-year-old former Ireland scrum-half Peter Stringer, responded by putting Quins under pressure, with giant centre Matt Banahan making inroads.
But the visitors suffered another setback when Quins winger Williams telegraphed a pass from Bath centre Kyle Eastmond on his own 22-metre line and raced 80 metres to touch down, leaving Botica another simple kick.
Bath opened their account six minutes later when their pack power proved too much for the home side.
Winning a lineout, their experienced forwards drove Quins back over their own line before Louw grounded the ball and Heathcote converted.
Bath's attacking momentum was broken when livewire Dickson's dummy and break from his own 22 turned defence into offence.
Bath halted the breakaway but, after Guest was hauled down just short of the line in the next phase of play, Dickson picked up and squeezed between Dominic Day and Stringer to score Quins' third try, with Botica again adding the extras.
A penalty from Heathcote reduced the Bath deficit to 21-10 just before half-time and, 30 seconds into the second half, he kicked another after Quins were caught offside.
But Harlequins struck again to stay in control with their fourth try in the 53rd minute. Botica found skipper Luke Wallace, who broke out of a tackle and passed outside for Williams to run in his second try, again converted by Botica.
But the home team suffered a serious blow when left-wing Sam Smith was yellow-carded for disrupting a Bath attack from a blatant offside position.
Bath, capitalising on their one-man advantage stayed in touch when Abendanon burst through for his side's second try, converted by Heathcote, who landed his third penalty in the 63rd minute to reduce the gap to five points and set up a tense finale.
Just before Smith returned, Bath had prop Nathan Catt sin-binned for illegally disrupting a Quins attack and Botica kicked the resulting penalty to seal victory.
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