The Steelbacks suffered their sixth defeat in as many Vitality Blast home matches this season as they lost by 12 runs to Durham Jets.
Only Ben Duckett (40) and Seekkuge Prasanna (38) really got going as Northants were unable to back up Wednesday's seven-wicket success at Derbyshire Falcons on Friday night.
The Steelbacks had looked likely to limit the high-flying Jets to a meagre total, but a late charge helped the away side up to 174 for seven.
Sri Lankan spinner Prasanna picked up creditable figures of two for 22 when his side put the squeeze on Durham during the middle of the innings.
But the men in red and black did enough to register a competitive total - and it proved too much for the Steelbacks to manage as they lost wickets at regular intervals.
It was a 10th defeat in 12 North Group matches for David Ripley's men, who are next in action when Yorkshire Vikings travel to the County Ground on Thursday evening.
They had gone into the Durham game on a high and after a 30-minute rain delay they won the toss, opting to bowl first as the skies turned blue.
Durham delivered 17 runs from the first over, bowled by Rory Kleinveldt, but Nathan Buck tightened things up in the second.
And Brett Hutton really turned the screw in the third, dismissing Graham Clark for 16 as Kleinveldt took the catch.
But Durham refused to buckle during the powerplay, with experienced duo Tom Latham and Paul Collingwood finding regular boundaries.
The Jets reached the end of the first six overs on 59 for one, setting themselves up for a big total in the County Ground sunshine.
But Latham was unable to deal with Prasanna, who bowled the Durham opener for 33 during an over that went for just two runs.
And it got even better for the Steelbacks in the next over as Ben Sanderson had Collingwood caught behind for 26.
That meant Durham had two new men in the middle with their total have ticked over to 76 for three.
And Prasanna was pulling the strings for the Steelbacks, not only stopping the flow of runs but picking up wickets, too.
He trapped Ryan Pringle LBW for 13 in the 12th over and by the time the 13th was done, the Jets had registered just 26 runs in their past six overs.
Hutton dismissed Will Smith for six in the next over as Charlie Thurston took a fine catch running in from the boundary.
And there was another superb take in the 18th over as Ben Curran hunted down a high ball and took it on the dive to give Kleinveldt a wicket with his final ball.
Stuart Poynter was the man to depart for 31, just as Durham were starting to threaten a final flurry.
Buck got rid of Ryan Davies before the end, having the batsman caught behind for 21 in the final over.
And Durham finished on a decent 174 for seven, giving the Steelbacks some hope of securing a second successive victory.
But the home side lost an early wicket as Smith took the catch at backward point to help Chris Rushworth get rid of Ricardo Vasconcelos, who cut a frustrated figure as he departed with just a single run to his name.
Josh Cobb came in and looked to be continuing his fine form, but the County Ground was left stunned when England pace ace Mark Wood bowled him for 14.
There was a big let-off for Duckett when Nathan Rimmington lost sight of the ball in the air and failed to take a catch that would have given his team another crucial wicket.
The Steelbacks finished the powerplay on 37 for two but they were struggling to get any momentum.
Thurston tried to take the fight to Durham but he was caught on the boundary by Smith for 16 off the bowling of Collingwood.
The Steelbacks were relying on Duckett and Alex Wakely to steer them out of trouble, and Duckett just managed to get the ball over Rushworth, who was agonisingly close to a catch.
But Wakely wasn't so fortunate as he was caught behind off Collingwood for 16 to leave the Steelbacks on 85 for four, needing 90 runs from the final eight overs.
And Duckett went soon after as he was trapped LBW by Smith for 40.
Prasanna put up a fight, blasting three sixes in his 38 from 18 balls, but when he was caught on the boundary off Nathan Rimmington, the Steelbacks innings fell apart.
Ben Curran and Rory Kleinveldt departed from successive deliveries off the bowling of Rushworth, who wasn't able to claim his hat-trick.
Brett Hutton also went before the end, losing his wicket for eight in the final over, bowled by Rimmington.
And Durham saw the game out to earn the victory.
Steelbacks: Vasconcelos, Duckett (wk), Cobb, Curran, Wakely (c), Thurston, Prasanna, Kleinveldt, Hutton, Buck, Sanderson.
Durham Jets: Latham (c), Clark, Collingwood, Smith, Davies, Poynter (wk), Pringle, Rimmington, Wood, Rushworth, Whitehead.