Cannot play media. Sorry, this media is not available in your territory.
Highlights: Northampton 33-20 Scarlets
Northampton (13) 33
Tries: Ashton 2, Reihana, Lamont Cons: Myler 2 Pens: Myler 3
Scarlets (13) 20
Tries: King, Stoddart, Davies
Cons: S Jones
Chris Ashton scored twice on Saturday to help Northampton into the semi-finals of the EDF Energy Cup.
The Scarlets went 10-0 up through tries from Regan King and Morgan Stoddart before Ashton grabbed his first.
Ashton's second try and a penalty apiece from Ceiron Thomas and Stephen Myler made it 13-13 at half-time.
Northampton stretched clear with tries from Bruce Reihana and Sean Lamont, with Jonathan Davies claiming a late consolation for the Scarlets.
Northampton are the second side through to the last four, along with Cardiff Blues, and the semi-finals will take place as a double-header at the Ricoh Arena in Coventry on 28 March 2009.
The match started in a flurry of tries, with three coming in the first 11 minutes at Franklin's Gardens.
Ceiron Thomas missed an early penalty but made amends when he sliced through the Northampton defence to set up the first try.
The Scarlets fly-half was hauled down but the Welsh side twice recycled possession and when the ball went wide King dummied his way over.
The visitors rapidly doubled their advantage as a swift tap and go from a penalty on halfway saw Stoddart set clear down the left.
An outside swerve saw the full-back beat the cover defender and scorch over in the corner for a cracking score.
Thomas was unable to convert either try and also missed a penalty and Saints soon hit back from a short-range line-out as Ashton darted over down the blind-side of the resulting ruck.
Myler missed the conversion and a subsequent penalty but both fly-halves did manage to find the target with a penalty apiece to make it 13-8 to the visitors just over halfway through the first half.
Northampton levelled the scores when Reihana gave Ashton the chance to squeeze in at the corner - albeit the last pass looked suspiciously forward.
Myler was once again off-target with the conversion and with Thomas falling short with a long-range penalty attempt the half ended all-square.
Northampton went in front five minutes after the restart as Reihana went over for a converted try.
Myler then missed the chance to extend Northampton's lead as he failed with a penalty.
But the Scarlets twice failed to add to their score as first David Lyons lost the ball just short of the line in a double tackle and then Stephen Jones - on as a replacement - hooked a penalty wide.
Myler finally succeeded with a penalty with 20 minutes to go to put Saints 10 points clear and Lamont's converted try put further daylight between the sides.
Davies crossed out wide to give the Scarlets a glimmer of hope but Myler's late penalty squashed any thoughts of a Welsh fightback.
Northampton boss Jim Mallinder:
"We had a shocking start to go 10-0 down but we'd expected it.
"But our defence improved significantly and it was good to go on and score some good tries.
"We're not the finished article but to come back from points down was good, to keep our composure and keep that lead."
Scarlets boss Nigel Davies:
"We're certainly not getting the bounce of the ball at the moment but I was proud of the way the boys played today.
"They gave everything they had but we let ourselves down on a couple of occasions.
"There's a couple of factors which affect our game. Discipline is a big problem for us and on occasions we leak some soft tries as well."
Northampton: Reihana; Ashton, Mayor, Downey, Lamont; Myler, Foden; Smith, Shields, Murray, Fernandez Lobbe, Lawes, Hopley, S Gray, Wilson.
Replacements: Diggin for Reihana (58), Dickson for Foden (53), J Gray for Shields (71), Tonga'uiha for Murray (71), Day for Lawes (22), Easter for Hopley (71).
Not Used: Spencer.
Sin Bin: Downey (70).
Scarlets: Stoddart; Higgitt, King, G Evans, M Jacob, C Thomas, Martens, I Thomas, Owen, Meeuws, D Jones, Day, N Thomas, Easterby, Lyons, G Thomas.
Replacements: Davies for Higgitt (54), S Jones for C Thomas (47), Roberts for Martens (51), Rees for Owen (47), John for Meeuws (67), G Thomas for N Thomas (47), Turnbull for G Thomas (77).
Ref: Wayne Barnes (England).