Somerset 360 for 7 (Trego 100, Myburgh 71, Hildreth 56) beat Hampshire 356 for 9 (Vince 109, Alsop 95, Dawson 76, Overton 3-69) by three wickets
South Africa pace bowler Dale Steyn had an uncomfortable return to competitive cricket for the first time since the Cape Town Test against India in January.
Steyn conceded 80 runs in 10 overs on a run-strewn night at the Ageas Bowl as Hampshire's 356 for 9 was overhauled by Somerset on the last ball of the match,m but at least had the relief at getting through unscathed only a few days after arriving in England for a short county stint.
Somerset's satisfaction over registering their record List A chase was shortlived, however, as they failed to qualify for the play-offs thanks to Essex's defeat of Kent at Chelmsford. Hampshire, meanwhile, still topped the group despite their three-wicket defeat, giving Steyn the chance to stick around and make an impact in a home semi-final later this month.
Steyn regularly hit speeds around the mid-80s, satisfying enough after his return from a heel injury which wrecked his Test comeback at Newlands. But he had a tough return, his second over leaking 18 as fellow South African Johann Myburgh, a big front footer, struck him for four boundaries, including three audacious leg-side blows off length balls.
Steve Davies also took him for 18 off an over, a simple dropped catch by Tom Alsop, who suffered a black eye as he failed with three grabs at the ball in the covers, adding to Steyn's frustration.
James Vince, Hampshire's captain, who had produced a sublime century earlier in the day, offered a sympathetic judgment, recalling accurately enough that Steyn's rhythm gradually returned as the match progressed.
"I thought he bowled well especially at the end," Vince said. "Up front he went for a few but also had a few dropped off him which could have affected the way he went.
"He only got off the plane a couple of days ago so hopefully he only gets better and better the longer he stays with us. He is a great bowler in all formats. He is a great asset to have and hopefully builds on today."
Peter Trego scored a classy century for Somerset, needing exactly 100 balls to collect his ninth List A ton but around the time he was dismissed it became clear that Somerset's route to the play-off was killed off.
Vince was in consummate touch until Lewis Gregory switched around the wicket to york him, impressive enough to encourage thoughts that he might yet become part of England's World Cup plans next summer. He was supported by Tom Alsop's bullish 95 and a carefree 76 not out from 47 balls from Liam Dawson as Somerset were faced with Hampshire;s highest List A score on their home patch.
But Myburgh and Davies got Somerset's chase off to a stormer as they smashed 87 off the opening 11 overs. Davies was also dropped by Mason Crane in the covers before Brad Taylor found surprising turn from a benign surface and bowled him with his third ball.
Myburgh was dropped on 48 by Chris Wood, before reaching his half century in 39 balls. The below-par fielding continued to help the visitors as Peter Trego was put down by Wood before the latter held onto a skier from Myburgh.
Hildreth departed after a rapid half-century, smartly stumped by Lewis McManus off Wood, but Trego continued to reach three figures before he struck Dawson to long off with eight overs remaining.
Steyn struck in his final over - the penultimate over of the match when Roelof van der Merwe holed out at long off. But Wood failed to defend 11 with Taylor dropping Craig Overton off the penultimate ball, before the tail-ender clipped over the top to win off the final delivery.
Steyn was not the only "debutant" to have a rough time. England's new spinner Dom Bess, fresh from his first two Tests against Pakistan, was playing his first List A game for Somerset and Vince and Alsop both exacted punishment as three overs cost 34.