Hampshire 320 for 8 (Prest 109, Lehmann 75, Bepey 3-64) beat Yorkshire 270 (Luxton 64, Ashutosh 3-48) by 50 runs
Hampshire repeated last September's Metro Bank One-Day Cup semi-final win over Yorkshire at Scarborough, impressively defending a 321-target to secure an opening day Group B triumph by 50 runs.
Tom Prest's superb 94-ball 109 underpinned a 320 for 8 - also including Jake Lehmann's 75 - for last year's beaten finalists, whose inexperienced attack fought back tigerishly from a fast White Rose start.
Indian overseas debutant Ashutosh Sharma struck twice in an over with his seam and finished with 3 for 48 from nine overs in Yorkshire's 270 all out.
Will Luxton and George Hill contributed 64 and 61 for Yorkshire, but the hosts needed more amidst a number of soft dismissals.
Lehmann and Prest united at 68 for 3 in the 17th over following early wickets for Ben Coad, Ben Cliff and another seamer in Isaac Bepey, a 25-year-old left-arm debutant from Sussex who has impressed playing for Leeds/Bradford University and Yorkshire's second team.
Bepey added two late wickets, but the innings had been transformed by Lehmann and Prest.
They shared a List A fourth-wicket record partnership on this ground, with Lehmann dropped on five and Prest on 73.
Prest was more dynamic, hoisting four sixes. Two went down the ground against spin and seam en-route to an 89-ball century, the 23-year-old's third in this format.
Hampshire's acceleration was shown in Hill's figures. He took the new ball and only conceded 10 runs in his first seven overs. His last three went for 43 and included the wicket of Prest, caught in the deep late on.
On a true pitch against an inexperienced bowling attack, Yorkshire would not have been daunted by the size of their chase. And Adam Lyth and Will Luxton took 46 off the first five overs.
The latter then hit four boundaries off Dom Kelly's seam in the seventh over - 71 without loss - and the hosts were motoring.
But two wickets fell with the score on 84 - added to another on 113 - to raise Hampshire hopes.
Lyth was lbw for 35 reverse sweeping at Andrew Neal's left-arm spin before Will Bennison was run out backing up without facing a ball. Luxton had reached his fifty off 36 balls but pulled debutant seamer Oli Williams down to long-leg.
Sam Whiteman (44) and Hill shared a calming 68, but nerves were jangling again when the former fell caught off a top-edge against Sharma with 181 on the board in the 30th over.
Hill lofted Harry Petrie over long-on for six en-route to his fifty off 55 balls but skewed a top-edge to short third against Neal, leaving the score at 233 for 6 after 39. Fin Bean had also been run out.
Every time Yorkshire looked like getting on top, Hampshire struck.
And when Sharma had Harry Duke caught at deep midwicket pulling and then captain Dom Bess caught at cover in the 43rd over, Yorkshire were 249 for 8.
Prest struck with his off-spin before Bepey was the third batter run out to wrap things up.
