Somerset, Twenty20 champions in 2005 and runners-up in three of the last five years, had lost their last three matches in the competition but a fine all-round performance from Peter Trego helped them end that run against in-form Hampshire at Taunton.
Trego, who turned 33 on Thursday, followed up his 94 not out against Glamorgan last time out with a 48-ball 76 as Somerset ran up a daunting 176 for four batting first.
Trego then claimed the wickets of James Vince and Jimmy Adams cheaply to leave Hampshire having to play catch-up in their run chase.
And with 30 runs needed off the last eight balls, a home victory looked assured, only for Will Smith to set Somerset hearts racing as he clubbed three sixes in a quickfire 25.
With Smith’s heroics, the target came down to four needed off the last two deliveries, but Alfonso Thomas kept his cool to earn Somerset the victory and inflict only a second defeat in six games on 2010 and 2013 winners Hampshire.
In the evening’s other early start, Warwickshire defended a total of 156 for five to defeat Nottinghamshire by 18 runs at Trent Bridge.
England duo Alex Hales (30) and Michael Lumb (24) looked to be powering Nottinghamshire to victory as they put on 57 in just 5.1 overs for the opening wicket, but after both fell in the same Shoaib Malik over the hosts badly lost their way as they slumped to a second loss in the North Division.
Laurie Evans earlier helped salvage a decent score for Warwickshire with an unbeaten 45 off 23 deliveries after Nottinghamshire all-rounder Samit Patel (three for 26) had shone with the ball.