Richard Levi smashed a stunning 88 from 43 balls - but the Steelbacks were beaten in a rain-affected game at Trent Bridge.
Nottinghamshire Outlaws recorded a 12-run win on the Duckworth-Lewis method to leave Northants frustrated.
On the previous evening, the weather had helped the Steelbacks, who won by two runs on the D-L method at Leicestershire Foxes.
But things evened themselves out inside just 24 hours as Notts' flying start - they were 52-0 when rain stopped play after 5.1 overs - earned the hosts a welcome win.
Only five overs were needed in the reply to constitute a game and the Outlaws just managed to get enough of their innings as Alex Hales (30no) and Rikki Wessels (14no) ensured it would be their team who took the spoils.
It was irksome for Northants, who had racked up 195 for eight after being put into bat, with Richard Levi the star of the show.
The big-hitting South African batsman smashed a superb 88 from just 43 balls, including seven fours and eight sixes, as the champions took control early on.
But Notts fought back well with the ball and the bat, meaning that they had the edge when the weather intervened.
The Steelbacks remain second in the North Group with three wins from their opening six matches and they will now prepare to host Worcestershire Rapids on Thursday night.
David Ripley's men were keen to win at Trent Bridge, which is a ground they have not been victorious on in a 20-over match.
And after skipper Alex Wakely lost the toss, the Steelbacks were put into bat, just as they had been at Leicestershire on the previous evening.
Samit Patel bowled a tight first over, with Levi managing just one four from it as the Steelbacks registered five runs.
The second over was similarly quiet, but Levi then got into his stride in the third, smashing a four and two sixes off the bowling of England man Jake Ball.
The Steelbacks were going at a rapid rate, with Adam Rossington also in the runs, but he was to go from the final ball of the fifth over as he was trapped LBW by Ish Sodhi for 17.
That brought Ben Duckett to the crease, with the batsman aiming to emulate his heroics of finals day last year, when he struck a superb 84 against Notts in the semi-final success.
Duckett introduced himself with a four and Levi continued to go well at the other end as the Steelbacks reached 63 for one at the conclusion of the powerplay.
Levi took a blow to the head in the eighth over and with him only having returned from concussion on the previous evening, there was a little concern around.
The ball had flown over the wicketkeeper for four and Levi was able to shrug off the impact as he looked to continue his fine form.
And Levi was to reach his half-century from just 26 balls with successive sixes, bringing up the Steelbacks' ton in the process.
Nottinghamshire were desperate to stem the tide and after Steven Mullaney conceded just three runs from his first over, Luke Wood bowled Duckett for 28.
The Steelbacks sent Rory Kleinveldt in at four in a bid to keep the pressure on, but the all-rounder went first ball to give Wood a hat-trick chance.
Wakely saw it out, but Nottinghamshire now had their tails up and Levi escaped as his sliced shot fell short of two advancing fielders.
Notts did think they had Levi when Mullaney took a catch on the boundary, but Wood had bowled a no-ball and the Steelbacks star escaped on 81.
But Levi did go in the next over, skying it and being caught by Hales off the bowling of Sodhi for a magnificent 88.
And Sodhi struck again in his next over, having Rob Keogh caught behind for eight as the Steelbacks innings started to stall a little.
Wakely hit a six off Patel before the bowler missed a sharp caught and bowled chance from the Steelbacks skipper.
But Wakely was to go for 19 in the next over as Wood took the catch off Harry Gurney and the Outlaws had fought back, leaving their opponents 170 for six from 17.1 overs.
Steven Crook came in and lashed a six, but Rob Newton was out for eight as he top edged a Gurney delivery and wicketkeeper Tom Moores took an easy catch.
Crook then found third man to go for 13, but Ben Sanderson finished the innings with a flourish, smashing Gurney for six off the final ball to set Notts 196 to win.
Sanderson then produced a tidy first over with the ball, conceding just two runs from it.
However, Notts got lift-off in the second, taking 16 runs off Richard Gleeson and then 13 off Kleinveldt in the next.
They hosts continued to motor on and when the rain started to come down, they had no issue with heading off as they were well ahead, earning them the victory.
Nottinghamshire Outlaws: Wessels, Hales, Patel, Christian (c), Taylor, Mullaney, Moores (wk), Wood, Sodhi, Ball, Gurney.
Steelbacks: Levi, Rossington (wk), Newton, Duckett, Wakely (c), Keogh, Crook, Kleinveldt, Sanderson, Gleeson, Shamsi.