Delray Rawlins hit a superb unbeaten 35 as Sussex secured a controversial three wicket victory over Gloucestershire at the Bristol County Ground with two balls remaining.
Gloucestershire players protested in vain when they incurred a six-run penalty for a slow over-rate, leaving the visitors to score seven off the final over.
Rawlins then kept his cool to knock off the winning runs, finishing with five fours and a six from a 17-ball innings which ultimately proved decisive.
Ian Cockbain had earlier scored 61 not out, sharing in stands of 67 and 38 with James Bracey and Benny Howell for the third and sixth wickets respectively as the hosts raised 159-6 after being put in.
Having successfully chased to beat South Group leaders Kent by nine wickets in their previous outing, Sussex again elected to field first. That looked a good decision once new-ball bowlers Ollie Robinson and Reece Topley accounted for openers Miles Hammond and Michael Klinger in quick time as Gloucestershire's top-order again fell short of expectations.
Gloucestershire enjoyed a much-needed stroke of luck when James Bracey was dropped at cover point on one by Rashid Khan off the bowling of Chris Jordan. He certainly made good his escape, posting 30 from 26 balls and staging a restorative stand of 67 in 9.1 overs with Cockbain for the third wicket.
Sussex deployed spinners Khan and Danny Briggs in tandem in an attempt to stem the flow of runs, a ploy which accounted for Bracey and the dangerous Ryan Higgins and caused Gloucestershire to lose crucial momentum during the middle overs.
Heavily dependent upon Cockbain, Gloucestershire were indebted to the Liverpudlian, who punished anything short of a length in raising 50 from 41 balls to keep his side in the game.
When big-hitting Jack Taylor missed a straight delivery and was pinned lbw by Jordan in the 16th over, it was left to Benny Howell to provide late acceleration, the veteran all-rounder scoring at two runs a ball in a valuable sixth wicket stand of 37 before holing out to long-on off Robinson in the final over.
Cockbain finished unbeaten on 61 from 49 balls, with 5 fours and a six, but not even his innings could dispel the feeling that Gloucestershire had fallen short of requirements.
But Sussex found the going equally difficult on a two-paced pitch, Philip Salt and Laurie Evans succumbing to David Payne and Tye respectively as Gloucestershire made good use of the new ball.
Sussex remained favourites until skipper Luke Wright, having compiled a painstaking 24, lost patience and holed out to long-on off the bowling of Tom Smith. 65-3 became 87-4 in the 13th over when Australian international Alex Carey played across the line and lost his off stump to Chris Liddle.
Dropped by Cockbain at backward point on 11, David Wiese threatened to dig the visitors out of a hole until falling lbw to Payne for 37 with the score on 126 in the 17th over.
Tye then returned to dismiss Khan, while Jordan holed out to long-on in the penultimate over as Payne finished with 3-35. But the six-run penalty ensured Sussex were favourites going into the final over and Rawlins needed no secind invitation.
Sussex captain Wright said: "We are always reminded about the rules regarding time before every game and you could see that the clock had just gone over the deadline. But we might have won anyway, even without the help of the six penalty runs.
"Delray Simpson struck the ball really well, especially on a pitch which was not the best. That's what they do here (in Bristol) and you know what to expect before you go out there. It's just as well that we bat all the way down, because we needed to.
"I thought our performance in the field was average, but to win when not playing at our best was pleasing. We have momentum and we're in a decent position in the group."
THE TURNING POINT
The decision by umpires Ian Gould and Mike Burns to impose a six-run time penalty on Gloucestershire at the end of the penultimate over, proved pivotal.
DELIVERY OF THE MATCH
Playing on Michael Klinger's perceived weakness against the short ball, Reece Topley produced a sharply rising delivery to have Gloucestershire's best batsman caught behind.
Ian Cockbain, whose disciplined innings of 61 not out, was overshadowed by what followed.