Somerset 172 for 9 (Banton 77, Smeed 63*, Currie 4-31) beat Hampshire 165 for 8 (de Grandhomme 66, Gregory 3-46) by seven runs

At the highest-scoring T20 ground in the world defending a total of 172 is near-impossible, not least with an in-form Colin de Grandhomme hitting 66 off 34 balls in a run-chase. But an extraordinary fightback and a dramatic collapse meant Somerset did just that to kick their Vitality Blast season into life and leaving Hampshire, once a proud T20 force, facing a fourth successive group-stage exit.

Hampshire needed 53 off 36 balls when Lewis Gregory, Somerset's captain, returned for his third over of the night, and when he conceded 18 off the first five balls with de Grandhomme cutting him away with disdain, it looked as though the game was up. But Gregory went round the wicket to angle one across de Grandhomme, who holed out to deep cover; Hampshire lost their heads, and five for 27 off the last 31 balls. When Chris Wood skewed a catch to cover off the last ball, Gregory roared in celebration: Somerset had a win in front of a home crowd under the Friday-night lights for the first time in two years.

This was the second-lowest 20-over total that Somerset had ever defended at Taunton, two runs more than the 170 they had managed against a Gloucestershire side captained by Kane Williamson a decade ago. They were reliant on Tom Banton and Will Smeed's half-centuries - they were the only two Somerset batters to reach double figures - but it was Marchant de Lange, recalled after missing the win at The Oval on Wednesday night, who was the real match-winner.

This was not the flat track that is usually served up here. "There was a lot more grass and the bounce was a bit tennis-bally," Banton said. "[de Lange] didn't play the last game and was pretty upset but he showed his class." He didn't concede a boundary in his four overs, defending the short leg-side boundary to the right-handers, and started the squeeze in the middle of the innings to keep the game alive.

It had looked for all money like it would be de Grandhomme's night, only half an hour before he was sat in the makeshift away dressing room listening to a long enquiry into Hampshire's fourth defeat in five completed games. He had taken the wickets of James Hildreth and his New Zealand team-mate Devon Conway, both chipping to mid-on in the second over - "I know how to bowl to him," de Grandhomme smiled wryly afterwards - and while Banton had dished out some rough treatment in his last two overs, he was in the mood with the bat.

De Grandhomme had driven straight down the A303 this evening from the Ageas Bowl, having taken a recovery day in the on-site hotel following New Zealand's World Test Championship final win. "I had a good rest yesterday all day," he said. "We had a good night, but then had a good day yesterday in the pit." He hit 18 runs off his first four balls, including one six off Craig Overton that cleared both pavilions at midwicket and must rank among the biggest on this ground, and tucked into Gregory's first two overs.

Wearing a shirt bearing only his initials - perhaps Hampshire's kit supplier charges by the letter - he put on 78 with D'Arcy Short for the second wicket and 44 with Joe Weatherley for the third, but with Liam Dawson absent on England duty, the rest of the middle order was unable to get Hampshire over the line. The decision to leave Sam Northeast out of the matchday squad for the last two weeks looks ill-advised at best, though at least their seven-match stretch of away games has finally come to an end.

The win was set up by Banton, whose 77 was the second-highest score of his T20 career and a welcome return to form after a difficult year for him. He was the aggressor in a 58-run stand with Smeed - three years his junior in a boarding house at King's College Taunton - and played his full range of shots, tucking into de Grandhomme over the leg side and reverse slog-sweeping Mason Crane for six.

"In previous games [this season] I've just gone too hard, too early," he said. "I'm quite emotional talking because that means the world to me to win that game. I've come back off a weird 12 months. It's been hard for me - I'm not going to lie and I'm not going to hide behind that. It's one of the worst games ever when it's not going for you."

Smeed, meanwhile, found scoring much harder early on, eking out 26 off his first 30 balls before finally kicking on towards the end of his innings, dragging Somerset towards something defendable and marshalling the tail - no mean feat for a 19-year-old. Scott Currie, who took 4 for 31, was the pick of the Hampshire attack, hanging the ball wide outside off stump and taking the pace off; less than two hours later, he was trudging off in defeat after two hopeful swings from No. 9.