The newly-formed part-time team, made up of the best players outside first-class cricket, were closing in on Sussex’s total of 255-8 with Wes Durston top scoring with 51.
But four wickets in just eight balls from veteran fast bowler James Kirtley prompted a collapse from 207-4 to 211 all out – 44 runs short of their target. Chris Nash hit 81 from 77 balls as Sussex celebrated a second Group A victory.
Lancashire also suffered a collapse, losing six wickets for just 20 runs, as they went down to a 42-run defeat to Somerset at Old Trafford.
Malaysian opener Arul Suppiah top scored with 80 as the Sabres rallied to 235 all out – Daren Powell taking 4-49.
The Lightning looked on course for victory with Paul Horton (59) and Stephen Moore (51) putting on 78 for the second wicket, only for Alfonso Thomas (4-34) and Ben Phillips (3-28) to turn the tables with a flurry of scalps.
Meanwhile, Steve Davies inflicted more pain on former county Worcestershire to help Surrey to a 30-run victory at New Road.
Davies, who hit a hundred in the County Championship meeting last month, hit 81 off 55 balls to help Surrey to their Duckworth-Lewis target of 191. Rao Iftikhar took 3-39 at Worcestershire made 235-7.
All-rounder Ryan ten Doeschate smashed an unbeaten century and claimed two wickets to help Essex to a five-wicket win over Derbyshire in another rain-affected match.