Somerset missed a chance to go top of Yorkshire Bank 40 Group C as they suffered a four-wicket defeat to Middlesex at Lord's.
A late charge from centurion James Hildreth negated a three-wicket burst from Ollie Raynor and saw the visitors post a total of 247-7, but Middlesex chased the target down with five balls to spare.
Dawid Malan was dismissed for the agonising total of 99 as the home side wobbled in the middle overs, but an unbeaten 39 from skipper Neil Dexter helped them across the line.
The Panthers made a good start to the chase as the opening pair put on 71 before Paul Stirling fell for 27 when he speared his 24th ball straight to mid-off.
Joe Denly scored 31 before he went for a rash big hit and was caught by wicketkeeper Alex Barrow, leading to a mini-collapse as Middlesex also lost Chris Rogers (1) and Adam Rossington (6).
When Malan drove wildly at the medium pace of Craig Meschede (3-53), and Barrow flung himself to his left to cling on to the edge, Somerset were right back in the game with their opponents on 185-5.
But Dexter added 44 in combative fashion with Gareth Berg (17), and Rayner also played well to finish on 12 not out as Middlesex made it across the line.
Hildreth had earlier completed his 102 in spectacular style, hitting Harris for three successive sixes and two fours in the final over of the Somerset innings.
Before the explosive end, Rayner's fine eight-over spell of 3-31 had put Middlesex well on top as he claimed the key wickets of Marcus Trescothick (41), Nick Compton (6) and Dean Elgar (15).
Trescothick and Peter Trego (35) had put on 73 before Rayner claimed his first scalp in the ninth over, but the innings started to stall thereafter.
Only Hildreth stood firm and the 28-year-old looked like as though he may have produced some match-winning brilliance with his magnificent late assault on James Harris.