Harlequins edged out Northampton with a 22-19 victory at The Stoop on Saturday to secure their play-off spot.
Both sides will have to travel away next weekend to advance to the Premiership final, with Northampton edging their way back into the match in the second half.
An aggressive opening ten minutes was closed out by Nick Evans' first penalty of the match, the rush defence from both sides forcing plenty of poor passing.
A key error in the Northampton defence then allowed Harlequins to burst into a convincing lead.
Danny Care spotted the space behind from a scrum and his bouncing grubber kick alluded Ben Foden, the ball popping up into the hands of Tom Williams who dotted down.
Northampton then looked to have hit back through Ken Pisi, the winger's carving run smoothly finding the space, but the try was ruled out by the TMO for obstruction.
Pisi's opposite wing Jamie Elliott however could not be denied, latching on to Foden's clever kick to dot down in the corner.
The visitors continued to enjoy the majority of possession, but their accuracy was sorely absent and it meant that by the interval, Harlequins remained out in front leading 10-5.
George Robson's lineout steal then set Harlequins on their way after the break, hooker Rob Buchanan spinning out of the tackle before grounding the ball against the base of the posts.
Evans missed a penalty to limit the damage on the scoreboard but Quins could not be held out for long, Lions reject Care sending a message to Warren Gatland.
His tapped penalty created the confusion and space for Easter's long pass to find Casson on the wing, dotting down in the right corner.
Northampton's pack notched up another score for Lions hooker Dylan Hartley thanks to a punishing maul, Myler's conversion moving the visitors back within ten points.
A loose second-half continued to present chances for Northampton who were more prolific after the break, Christian Day sealing their third try of the match and narrowing the gap.
Quins left a bonus point chance out on the field when Monye slipped after combining with Brown, the former Lions winger making a rare mistake in those kind of situations to leave the hosts only three points ahead.
Northampton however were unable to move ahead in the dying stages, finishing in fourth spot and preparing for a trip to face Saracens, whilst Harlequins must travel to Welford Road to face Leicester.
Tries: Williams, Buchanan, Casson
Con: Evans 2
Tries: Elliott, Hartley, Day
Cons: Myler 2
Harlequins: 15 Mike Brown, 14 Tom Williams, 13 George Lowe, 12 Tom Casson, 11 Ugo Monye, 10 Nick Evans, 9 Danny Care, 8 Nick Easter, 7 Luke Wallace, 6 Tom Guest, 5 George Robson, 4 Olly Kohn, 3 James Johnston, 2 Rob Buchanan, 1 Joe Marler.
Replacements: 16 Joe Gray, 17 Mark Lambert, 18 Will Collier, 19 Charlie Matthews, 20 Maurie Fa'asavalu, 21 Karl Dickson, 22 Ben Botica, 23 Ross Chisholm.
Northampton: 15 Ben Foden, 14 Ken Pisi, 13 George Pisi, 12 Luther Burrell, 11 Jamie Elliott, 10 Stephen Myler, 9 Lee Dickson, 8 Samu Manoa, 7 Tom Wood, 6 Calum Clark, 5 Christian Day, 4 Courtney Lawes, 3 Brian Mujati, 2 Dylan Hartley (capt), 1 Soane Tonga'uiha.
Replacements: 16 Mike Haywood, 17 Alex Waller, 18 Tom Mercey, 19 Phil Dowson, 20 GJ Van Velze, 21 Martin Roberts, 22 Ryan Lamb, 23 James Wilson.
Referee: Tim Wigglesworth
Assistant Referees: Andrew Pearce and Gareth Copsey
TMO: David Grashoff