A last-minute missed conversion from Shane Geraghty saw Saracens sneak a 28-27 victory over Northampton at Franklin's Gardens on Saturday.
Ben Foden gave Geraghty the chance to seal the win when he touched down late on but the replacement sprayed his conversion attempt wide and with it saw an end to Saints' unbeaten home record.
For Saracens victory ensured they would play a semi-final and after the fourth meeting between these sides this season, there is every chance a fifth may arrive in the play-offs.
Saints took a fifth-minute lead from the second penalty of the match when Stephen Myler, making his 100th appearance for Northampton, curled in a fine kick from wide on the left.
But Saracens hit back within a minute, Rodd Penney's pass sending captain Ernst Joubert in by the left wing flag, despite the attentions of Chris Ashton. Glen Jackson converted from the touchline.
Mouritz Botha fell foul of referee Chris White in the 12th minute, knocking forward a Northampton pass inside his 22 with the intention of intercepting. White considered it a penalty, and Myler won't have a simpler kick to execute for what remains of the season.
The early exchanges found Saracens under pressure in the scrum where they conceded two penalties for failure to stay up and engaged.
And the second penalty sparked a siege on their line that was only relieved by conceding another penalty, again whipped over the bar by Myler.
Saracens were awarded their first kickable penalty in the 23rd minute for Saints not moving clear of a breakdown after tackling.
Jackson made no mistake to recapture the lead, but Saints went straight back on the offensive, pinned the visitors inside their 22 again and extracted another penalty, this time for coming in at the side.
But this time, the usually reliable Myler hammered the ball almost too hard, and watched it climb high and wide of the goal.
Northampton enjoyed the lion's share of ball, but the capacity Franklins Gardens crowd were seeing, not for the first time this season, the meanness of Saracens defence close to their goal-line.
The game erupted in the 33rd minute when Saracens halted an attack on the right by tackling Ashton just before Bruce Reihana's pass reached the prolific wing. White sent Botha to the sin-bin and awarded a penalty-try, converted by Myler.
Feelings were running high, and when Courtney Lawes hit Penney late, the wing responded by grabbing the flanker's ankle and toppling him.
The two club chairman may well have come to an 'amicable' resolution to the Soane Tonga'huita affair - the prop signed a contract with Saracens for next season only to change his mind after three weeks and sign a new deal with Saints - but there was clearly an edge to proceedings on the field.
Saracens reduced the arrears straight from the re-start.
Brad Barritt's break put Saints on the back-foot, they offended at the contact-area, and Jackson's penalty was spot on.
Northampton were handed the opportunity to restore their six-point lead on 45 minutes and Myler's accuracy returned, his kick from the right sailing high between the sticks.
Saracens scored a tremendous try two minutes later.
Centre Adam Powell crossing on the left following a multi-phased passage of play in which Jacques Burger and Jackson played key roles. Jackson's pass sent Powell in for the score after which he added the conversion.
Jackson then fired a penalty wide in the 52nd minute.
Some of Saracens attacking play was breathtaking, even in the eyes of home die-hards, but that crucial second score was not going to be earned without a massive additional effort against a team defending a 100% home league record.
And it was Saints who struck next, Myler kicking his sixth successful effort of the match on the hour to edge them 22-20 ahead - only for Saracens to spark the seventh lead change with a Jackson penalty after 63 minutes for offside.
Saracens' adventure was rewarded after 70 minutes when, after battering Saints on their line, the ball was whipped wide left where Penney managed to squeeze in by the flag. Jackson's conversion-attempt went wide.
Saints struck in the 78th minute after a succession of attacks when Foden raced over - but replacement Geraghty missed the conversion.
Tries: Penalty, Foden
Pen: Myler 5
Tries: Joubert, Powell, Penney
Con: Jackson 2
Pen: Jackson 3
Northampton: 15 Ben Foden, 14 Chris Ashton, 13 Jon Clarke, 12 James Downey, 11 Bruce Reihana, 10 Stephen Myler, 9 Lee Dickson, 8 Roger Wilson, 7 Phil Dowson, 6 Courtney Lawes, 5 Juandre Kruger, 4 Ignacio Fernandez Lobbe, 3 Euan Murray, 2 Dylan Hartley (capt), 1 Soane Tonga'uiha.
Replacements: 16 Brett Sharman, 17 Regardt Dreyer, 18 Brian Mujati, 19 Mark Easter, 20 Neil Best, 21 Alan Dickens, 22 Shane Geraghty, 23 Joe Ansbro.
Saracens: 15 Alex Goode, 14 Michael Tagicakibau, 13 Adam Powell, 12 Brad Barritt, 11 Rodd Penney, 10 Glen Jackson, 9 Neil de Kock, 8 Ernst Joubert, 7 Andy Saull, 6 Jacques Burger, 5 Mouritz Botha, 4 Hugh Vyvyan, 3 Petrus du Plessis, 2 Schalk Brits, 1 Matias Aguero.
Replacements: 16 Ethienne Reynecke, 17 Rhys Gill, 18 Richard Skuse, 19 Tom Ryder, 20 Justin Melck, 21 Justin Marshall, 22 Derick Hougaard, 23 Kameli Ratuvou.
Referee: Chris White