Wasps 19-10 Northampton
Wasps struggled to pull clear
Wasps booked their place in the Zurich Premiership final against Gloucester on 31 May with an entertaining victory over Northampton at Adams Park on Saturday.
The Londoners were in command from the first whistle but a combination of missed chances and Northampton's refusal to admit defeat meant they could not relax until the very end.
Fraser Waters' try in first-half injury time looked likely to open the floodgates but Ben Cohen's reply for Saints within two minutes of the restart tightened the game up once more.
Although Wasps had further chances, they still needed the boot of Alex King to see them home, the stand-off landing three penalties, a drop goal and conversion.
Northampton took the field missing four key players - Budge Pountney, Matt Dawson, Tom Smith and Peter Jorgensen - and lost another when Paul Grayson was forced off by an ankle injury with just 22 minutes played.
Wasps (13) 19:
Pens: King 3
Saints (3) 10:
At that point, they were only three points behind, Wasps' domination of the opening quarter having yielded only a solitary penalty from King.
The hosts' frustration at the poor handling and decision-making that prevented them turning possession into points lasted almost until the break, when they finally crossed the Northampton line.
Having answered a Bruce Reihana penalty with another of his own, King capitalised on a Northampton mix-up to hack ahead in midfield.
First Cohen and then Nick Beal failed to deal with the loose ball, allowing Waters to dive over for the try and giving King a simple conversion for a 13-3 half-time lead.
That should have settled Wasps down but Saints started the second half at breakneck pace.
Cohen confirmed his reputation as one of the game's sharpest finishers, standing up Josh Lewsey by sleight of foot before powering over at the left flag.
Reihana converted from the touchline to close the gap to three and herald a spell of hard running and harder tackling from both sides.
But it was Wasps who emerged on top, as King kicked the penalty and drop goal that ultimately won the match.
Saints, in contrast, lost Mattie Stewart to the sin-bin and the influential John Leslie and Andrew Blowers to injury.
Wasps: Van Gisbergen; Lewsey, Waters, Abbott, Logan; King, Howley; Dowd, Leota, Green, Shaw, Birkett, Worsley, Volley, Dallaglio (capt).
Replacements: Wood, Greening, Rudd, Erinle, Lock, Scrivener,
Saints: Beal; Ripol, Reihana, Leslie (capt), Cohen; Grayson, Vass; Stewart, Thompson, Morris, Lord, Hunter, Connors, Fox, Blowers.
Replacements: Miles, Budgen, Williams, Seely, Brooks, Tucker,
Referee: Chris White (RFU)