Gloucestershire 151 for 8 (Hammond 63) beat Middlesex 148 for 9 (Simpson 42*) by two wickets
Cheltenham-born Miles Hammond delighted his home crowd with a brilliant array of shots as Gloucestershire inflicted a first defeat on Middlesex in this season's Vitality Blast.
In front of a sell-out 5,000 attendance at a steaming College Ground, the 23-year-old opener smashed 63, including nine fours and three sixes, to help see his side to a two-wicket win after the visitors had won the toss and elected to bat.
Without AB de Villiers, who was nursing a hand injury, Middlesex could post only 148 for 9 on the fast-scoring College Ground, Stevie Eskinazi making 40 and John Simpson 42 not out. David Payne claimed 3 for 32, while Ryan Higgins and AJ Tye both returned 2 for 23.
In reply, Gloucestershire slipped to 84 for 5, despite Hammond's efforts, before Benny Howell (33) and Jack Taylor (24) made sure they reached 151 for 8 and won with four balls to spare.
Middlesex began like a team who had won their first two group games, as Eskinazi hit three fours off the first four balls of the match, sent down by Payne, and Dawid Malan lofted Graeme van Buuren's second ball from the other end over deep square for six.
But left-arm spinner van Buuren stemmed the early momentum with his next delivery, which saw Malan caught and bowled off a skyer.
Dan Lincoln came in at three in the absence of de Villiers for his Middlesex debut. The 24-year-old Surrey-born batsman and Eskinazi, who faced only 26 balls, took the score to 73 before the latter was brilliantly stumped by James Bracey off a Howell leg-side wide.
Lincoln, a non-league goalkeeper who was only registered by Middlesex yesterday, made sure the momentum was maintained, hitting Higgins for a straight six, before being caught at deep midwicket off the following delivery for a promising 30.
The same over saw Nick Gubbins taken at long-on for a single and at the halfway stage Middlesex were 81 for 4. The visitors were then squeezed by Howell, AJ Tye, Higgins and Tom Smith, losing George Scott cheaply as they added 26 in four overs.
Toby Roland-Jones and Tom Helm perished to catches at extra cover off Payne, while Nathan Sowter also succumbed to an attempted big hit, and Mujeeb was bowled by Tye for a duck.
Middlesex had lost their early momentum, with a straight six by Simpson off Howell in the 15th over a rare blow of defiance.
Gloucestershire's innings began as their opponents' had done, Hammond hitting two fours and a six in the opening two overs.
But the third saw Tom Helm strike a double blow, having dangerman Michael Klinger caught behind for ten and Ian Cockbain taken at point two balls later to leave the hosts 25 for 2.
Hammond continued to attack with a series of sweetly-timed shots that help take the hosts to 59 for 2 at the end of the powerplay. Soon the local boy was raising his bat to acknowledge warm applause for a 26-ball fifty.
Bracey and Higgins fell cheaply before Hammond's superb knock was ended by a low catch from wicketkeeper Simpson off Roland-Jones. At 84 for 5, Gloucestershire had to regroup.
Howell, dropped at short cover before he had scored, and Taylor added 45 priceless runs together. Both fell at the end, along with van Buuren, but had already ensured a hugely successful festival for Gloucestershire ended in style, as Smith hit the winning runs.