Durham 297 (Stoneman 85, Mustard 65) beat Lancashire 258 (Clark 72, Onions 4-45) by 39 runs
Durham were given a late scare by Kabir Ali, making his Lancashire debut, before clinching a 39-run win in the Yorkshire Bank 40 match at Chester-le-Street.
The visitors were 162 for 8 in reply to 297 for 9 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 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 50 off 37 balls with ten 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 balls, and Ben 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 for 45.
Ashwell Prince sped to 22 before failing to clear mid-on, then 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 4, 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 ten 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.