Sussex 163 for 7 (Wiese 37, Rawlins 35*) beat Gloucestershire 159 for 6 (Cockbain 61*) by 3 wickets
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 for 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 Bracey was dropped at cover point on one by Rashid Khan off the bowling of Chris Jordan. He made good his escape, posting 30 from 26 balls and staging a restorative stand with Cockbain.
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 Howell to provide late acceleration, the veteran all-rounder scoring at two runs a ball in a valuable stand with Cockbain before holing out to long-on off Robinson in the final over.
Cockbain finished unbeaten on 61 from 49 balls, with five fours and a six, but not even his innings could dispel the feeling that Gloucestershire had fallen short of requirements.
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. When Australian international Alex Carey played across the line and lost his off stump to Chris Liddle, they were 87 for 4.
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 for 35. But the six-run penalty ensured Sussex were favourites going into the final over and Rawlins needed no second invitation.
Both sides wore shirts emblazoned with a rainbow in support of the Rainbow @ Grief Encounter charity, which helps bereaved children and their families, like Gloucestershire spinner Tom Smith's following the death of his wife, Laura, last August. Many players also wore rainbow face paint after Smith last week spoke of the support the organisation and the Professional Cricketers' Trust had given him and his two young daughters.