Durham Dynamos were given a late scare by Kabir Ali, making his senior Lancashire debut, before clinching a 39-run win over the Lightning in the Yorkshire Bank 40 match at the Emirates Durham ICG.
The visitors were 162 for eight in reply to 297 for nine when Kabir went to the crease to join Jordan Clark. They added 45 in four overs before Clark departed for a career-best 72; then Stephen Parry shared a last-wicket stand of 51 in six overs.
Kabir thrashed five sixes, four of them off Chris Rushworth, in his 24-ball half-century before he was bowled for 59 by Ben Stokes with the total on 258.
Lancashire were all out with 3.5 overs left and last year's semi-finalists now have little chance of progressing from Group B after being outplayed by Durham for the second time in this season's competition.
Simon Katich invited Durham to bat in perfect conditions and acting captain Mark Stoneman put on 126 in 14 overs with fellow opener Phil Mustard.
After playing second fiddle in the first few overs, Mustard hit five fours in an over from Clark to reach a fifty off 37 balls with 10 boundaries.
He holed out at long-on for 65 off Parry, but Stoneman went on to make 85, while Gordon Muchall followed his 30-ball half-century against Derbyshire on Monday by making 48 off 33 deliveries, and Stokes weighed in with 40 off 33.
Lancashire raced to 29 after four overs in reply but in the next five overs they lost four wickets, three of them to Graham Onions who bowled straight through his eight overs to finish with 4-45.
Ashwell Prince sped to 22 before failing to clear mid-on, and Stephen Moore drove Onions to mid-off.
Rushworth nipped one back to hit Karl Brown's middle stump and, when Onions bowled Katich for a duck, the game looked as good as over at 44 for four, but 22-year-old Clark kept Lancashire ticking over.
With a top score of 32 in seven previous List A appearances, he started slowly but hit 10 fours and a six in scoring his runs off 62 balls.
Gareth Cross aided the revival with 36 off 24 deliveries and Kabir's blitz had Durham rattled until a Stokes yorker took out his middle stump.