Somerset beat One-Day Cup holders Gloucestershire by one wicket with just three balls to spare at Taunton.
Chris Dent’s second List ‘A’ century (100) proved the Gloucester backbone.
In reply to a total of 260, an 87-run fourth-wicket stand between Johann Myburgh and James Hildreth then looked to have the hosts on course until six wickets went down for just 32 runs.
But Jamie Overton put on 65 for the last wicket with Tim Groenewald as Somerset claimed victory on 263-9.
Overton made 40 and Groenewald hit 34 in an unbroken stand of 65 off 49 balls, the second highest 10th-wicket stand to win a one-day match in England.
After the trophy holders were put into bat by their neighbours, Dent played exceptionally well whilst his teammates fell around him, reaching three figures off 107 balls before being caught at cover off Groenewald.
Spinner Roelof van der Merwe starred for the home side, running out Iain Cockbain as well as dismissing Benny Howell, Tom Smith and bowled Craig Miles off the final ball of Gloucestershire’s innings.
The visitors reduced their local rivals to 15-2 as Adam Hose and Peter Trego fell cheaply. But they rallied well until Myburgh picked out long-off off Dent to fall for 81 and begin a middle-order implosion.
Lewis Gregory was then superbly run out and Hildreth was caught and bowled by Smith in the next over as Somerset fell from 166-3 to 198-9.
But, with 63 still needed to win, Overton and Groenewald refused to go down without a fight in this local derby, finding the boundary with ease.
They survived one near miss when, with the hosts still needing 13 off 3.1 overs, wicketkeeper Gareth Roderick missed a golden opportunity to run out last man Groenewald.