Cobus Reinach scored twice as Saints booked their place in the Premiership Rugby Cup semi-finals beating Leicester Tigers 47-20 at Franklins Gardens.
Tigers managed to mount a fightback during the second half, but Reinach's double ensured Saints would secure the bonus-point win that earns them a place in the final four.
Chris Boyd's men had been cruising when Heinrich Brüssow bagged their fourth try eight minutes after the break.
But Tigers refused to go quietly and after scoring 10 points after some lengthy spells camped in Saints' half, anxiety levels started to rise in the home ranks.
But Reinach managed to wriggle free to give Saints some much-needed breathing space, and further tries from the South African No.9 and Tom Collins sealed the win.
The black, green and gold will now face a semi-final on the second weekend of February as they look to take another step towards earning their first trophy under boss Boyd.
They went into the game against Tigers in expectant mood, having named a strong side against largely inexperienced opposition.
But it was Tigers who took control from the off, penning Saints in their own half and applying real pressure with two penalties kicked to the corner.
And after the ball came to Fred Tuilagi, the big No.8 grabbed the opening score of the game, proving too strong for the Saints rearguard.
Joe Ford added the extras and there was a hush around the Gardens, with the home fans waiting for their side to spring into life.
And they didn't have to wait too long as Ken Pisi made the initial inroads and James Grayson eventually finished the move with a flourish, slaloming his way in from the right.
The scorer added the conversion to level things up and Saints had the bit between their teeth as they started to show real ambition.
Kickable penalties were continually turned down before hooker James Fish grabbed a trademark try from a lineout drive.
A typical derby-day scrap followed as proceedings started to heat up, but Grayson missed the conversion to leave his side's lead at five points.
It was extended to 12 eight minutes before the break thanks to a fine Saints score.
George Furbank kicked ahead and then applied real pressure to opposite number Jordon Olowofela, allowing Saints to win the ball back. Brüssow was then heavily involved with a superb long pass creating space on the left, where lock Dom Barrow showed real determination to register his first Saints try.
Barrow's effort came against his former employers, and Grayson rubbed salt in the wounds with the successful conversion.
Saints were now on the hunt for the bonus-point try they wanted, but Tigers were next to register points as Ford landed a penalty.
Another penalty chance soon arrived after Brüssow was penalised for hands in the ruck, but Ford fluffed his lines as Tigers remained nine points down.
That was how it stayed at half-time, but Saints emerged in determined fashion for the second period and they were to score the bonus-point try six minutes into the half.
After a period of prolonged pressure, Brüssow was the man on the mark, muscling his way to the line under the posts.
Grayson converted before Ford narrowed the gap with a penalty.
Tom Varndell almost managed to sneak in for a try on his return to Leicester action, but after the wing hunted down Jamie Gibson, the ball was held up.
From the resulting scrum, Saints got a good shove on, but Tigers managed to hold on to the ball and they started to crank up the heat in the set piece, winning penalty after penalty.
But with Ehren Painter up against former Saints prop Campese Ma'afu, the home side finally won a decision from the referee, helping them to escape unscathed.
James Haskell was then introduced for his return from injury, replacing the relentless Brüssow.
But having soaked up all the Tigers attacks, Saints were then hit with a sucker punch as Furbank saw his attempted kick ahead charged down and Ford raced away to score.
Ford converted and the gap was just six points with 11 minutes remaining as nerves started to jangle at the Gardens.
But Reinach brought relief as after Saints knocked loudly on the door, the scrum-half somehow managed to find space under the Tigers rearguard to score.
Grayson converted and Tigers were down and out, conceding two more tries before the end as Furbank sent Collins in out wide and Reinach put the icing on the cake by racing home from halfway.
Saints: Furbank; Pisi (Davies 76), Tuitavake (Kellaway 58), Hutchinson (Tuala 66), Collins; Grayson, Reinach; van Wyk (Davis 76), Fish (Marshall 60), Franks (Painter 56); Ratuniyarawa, Barrow (Moon 56); Gibson, Brüssow (Haskell 66), Harrison.
Leicester Tigers: Olowofela (Steward 76), Sarto, Aspland-Robinson, Hardwick, Varndell, J Ford, White; Gigena (Ma'afu 60), McMillan (Mahoney 75), Heyes (Feao 60); Lewis, Wells (c) (Gough 75); Reffell, Evans, Tuilagi.
Referee: Adam Leal