Sussex Sharks collapsed as Steven Crook took three wickets in 11 balls as Northamptonshire Steelbacks improved their chances of progressing in the Yorkshire Bank 40 with a 63-run win in Group A at Arundel.
After the Steelbacks made 191-6 from 39 overs, Crook laid waste to Sussex's top order by removing Chris Nash for one and Luke Wells and Ed Joyce for ducks. David Willey also picked up the scalp of Luke Wright as the Sharks were reduced to 7-4 in the sixth over.
Mike Yardy (52) and Ben Brown (40) gave Sussex hope but they were eventually bowled out for 128 in the 35th over.
Crook triggered the Sussex collapse with his fifth ball when Nash played on and in the next over Wright was startled by a ball from Willey that rose alarmingly and looped into the covers.
In his next over Wells had no answer to a perfect off-cutter from Crook then Joyce played all around a slower ball. Crook returned later in the innings to have Will Beer caught off a top edge to finish with 4-26.
When Rory Hamilton-Brown picked out mid-off in the 12th over Sussex were 32-5, but Yardy and Brown gave them respectability in a stand of 73 in 14 overs.
It was never easy to score on a slower pitch but Yardy struck five boundaries in a 66-ball 50 and Northants had to bring back Willey to end the partnership when he was bowled making room to hit through the leg side.
Brown, who spoiled Crook's figures by mowing him over mid-wicket for the only six, was bowled by Azhar Ullah who finished with 3-20 as Sussex's last four wickets fell for 11 runs.
Earlier, Alex Wakeley made 69 and 18-year-old Ben Duckett an unbeaten 47 in Northants' 191 after rain delayed the start by 15 minutes.
They added 73 in 14 overs for the fifth wicket after Sussex's spinners had inflicted early damage. Off-spinner Chris Nash took two wickets with his first eight balls, bowling Willey with his first delivery and trapping Kyle Coetzer leg-before.
Leg-spinner Beer also picked up two wickets during an impressive spell but Wakely made the most of a reprieve when he was dropped on 28 by Chris Liddle at mid on to make 69 off 87 balls before he was caught in the deep in the penultimate over.
Duckett, in only his third List A appearance, hit five boundaries in a run-a-ball 47 as Northants plundered 26 off the last two overs to set up a fourth win that lifts them to second place in the table.