Somerset overcame a top-order collapse to book their spot in the Vitality Blast final courtesy of an improbable two-wicket victory over Hampshire with two balls to spare at a rocking Edgbaston.

Chasing Hampshire's 150 all out, underpinned by Joe Weatherley's career-best 71 off 50 balls after he was given a major reprieve by fielder Marchant de Lange's lapse in the field, Somerset looked doomed on 34 for five.

While Tom Abell kept them afloat with an important 50 from 35 balls, they still needed 48 runs from the last 3.2 overs. But Ben Green's 35 off 18 gave them hope and Josh Davey followed up his four for 34 with the ball with a cameo 11 not out from three deliveries as Somerset sealed a place in the showpiece against either Kent or Sussex.

Hampshire were asked to bat first and were still in single figures when James Vince and Toby Albert departed to catches behind the wicket. Tom Prest was then bowled through the gate by a nip-backer from Davey, who thought he had his third wicket when Tom Banton took a steepler running forward after Weatherley's slog-sweep had caught the top-edge.

But Weatherley, who had already cleared deep square-leg twice with identical strokes, was wise to De Lange drifting outside the 30-yard circle in breach of fielding restrictions and gesticulated to the umpires. A no-ball was called, with Weatherley granted a major let-off on 26, and Liam Dawson hammering the extra delivery for six.

It was not an immediately obvious turning point, with slow left-armers Roelof van der Merwe and Lewis Goldsworthy subduing Weatherley and Dawson, who was cleaned up for 18 by a devilish yorker in Ben Green's only over.

Only two boundaries came in the 10 overs after the powerplay, with Hampshire on 99 for five at the end of the 16th. While wickets continued to tumble, they upped the rate in adding 51 from the final four overs as De Lange's miserable morning continued.

He was spanked for two sixes apiece by Chris Wood, who made an entertaining 18 off six, then Weatherley. Either side of that Davey returned to bowl Wood and Scott Currie before Weatherley holed out in the final over to give De Lange his only moment of success.

Somerset made a watchful start but capitulated from 15 without loss to 34 for five within the space of 22 balls, seemingly guaranteeing Hampshire would break their semi-final hoodoo, having lost four times at this stage since they last won the competition in 2012.

Banton picked out the deep midwicket fielder, Vince just about got his fingers underneath a chance at mid-on to see off Van der Merwe before taking a catch at mid-off to dismiss Will Smeed. Goldsworthy summed up the scrambled minds of Somerset's batsmen when he was run out while Tom Lammonby was trapped in front by Mason Crane first ball.

Some lusty blows from Abell kept the 23,500-crowd entertained but the asking rate was at nearly 14 an over when he holed out in the 17th over.

A seemingly hopeless cause was ignited when sloppy full tosses from Brad Wheal were taken the distance by Green then Overton while the former then crunched Wood twice over midwicket in the next over before holing out.

But Green's cameo meant Somerset needed just 10 from the last over, and his replacement Davey thumped Wheal down the ground for six before, with the field up, a leg-side four brought the 2005 champions victory.