Somerset 187 for 6 (Smeed 78) beat Gloucestershire 186 for 8 (Roelofsen 52, Taylor 42*) by four wickets
Will Smeed proved the scourge of Gloucestershire for the second time this season as Somerset guaranteed themselves a home quarter-final in the Vitality Blast with a four-wicket win over their neighbours at Taunton.
The visitors did well to recover from nine for three after losing the toss to post 186 for eight, Grant Roelofsen top-scoring with 52, while Jack Taylor (41 not out) and Graeme van Buuren (36) made significant contributions. Matt Henry was the pick of the Somerset attack with 2 for 28.
Somerset's reply of 187 for 6 in 19.3 overs was dominated by opener Smeed, who followed up his 94 in the corresponding South Group game at Bristol, by blitzing 78 off 42 balls, with five sixes and seven fours.
Ben Green (37) and Kasey Aldridge (32 not out) clinched victory with a sixth-wicket stand of 48 as the group leaders made it ten wins from 12 games, ensuring a top two finish as their closest pursuers, Surrey and Essex still have to play each other.
It seemed Gloucestershire would need a good start to stand a realistic chance of springing a surprise and they got exactly the opposite, losing Miles Hammond and Ben Charlesworth in Henry's first over, the second of the game.
When Craig Overton had Ben Wells caught behind off a lifter, Gloucestershire were in disarray. James Bracey hit a quick 17, including a scooped six off Overton, but he somehow contrived to sky a catch to point off what would have been a leg-side wide from Josh Davey and at the end of the six-over power play, the scoreboard read 44 for 4.
Roelofson and van Buuren then produced an excellent stand of 71 in 6.2 overs to revive Gloucestershire fortunes. The 11th over, bowled by the leading wicket-taker in the competition, Green, went for 20 as Roelofsen struck sixes over mid-wicket and square leg.
Offspinner Shoaib Bashir, making his home debut for Somerset, bowled van Buuren, who had hit two sixes and four fours, with a ball that turned, but Roelofsen went to fifty off 36 balls before being run out by Tom Abell's direct hit at the bowler's end attempting a single to extra cover.
Dale Benkenstein, Gloucestershire's head coach, suggested that Somerset should have recalled Roelofsen after he collided with Henry. "I didn't think it was a fair dismissal," he said. "I hold Tom Abell in very high regard and respect him as a cricketer, but I felt it was a very poor decision not to recall Grant.
"Their argument is that he should have run around the bowler and they were within their rights to appeal. But had the bowler not been there, Grant would have made his ground and I don't think it was fair cricket."
David Payne was also run out, at the non-striker's end, by some sharp fielding off his own bowling by Green, but Taylor began to enjoy himself, clearing the ropes off Green before being caught at third-man off a Henry no-ball.
The reprieve saw Taylor hit a second six, off Henry, and Josh Shaw also managed a maximum off Davey, who responded by bowling the last over of the innings for just three runs.
Shaw began the Somerset innings with a wicket-maiden, having Tom Banton caught behind off his second ball. But Smeed retaliated with a six and a four off the second over, sent down by Payne.
Smeed then hit Ajeet Dale over midwicket onto the top floor of the flats as 17 came off the third over. But Payne hit back in the next, Tom Kohler-Cadmore skying to mid-on where Gloucestershire skipper Hammond pouched a comfortable catch.
Abell was bowled attempting to scoop a full ball from Zaman Akhter and Somerset reached the end of the powerplay on 53 for 3, Smeed having effortlessly lifted another six over midwicket off Shaw.
Akhter then yorked Sean Dickson before Smeed went to a 26-ball fifty that thrilled another packed Cooper Associates County Ground crowd. His brilliant exhibition ended in the 13th over when he miscued a pull shot off Dale to van Buuren at long-on.
Green smacked a straight six off van Buuren's left-arm spin as the 14th over went for 16, then cleared the extra cover boundary rope off Shaw. Aldridge provided excellent support and 27 were needed off the last three overs.
Green fell to a catch at short fine leg off Dale. But Craig Overton's six over midwicket and straight four off Akhter left Somerset requiring eight off the last over, bowled by van Buuren.
An Aldridge four over extra cover was sandwiched between a brace of twos and sealed the deal to the delight of home supporters.
