Wicketkeeper John Simpson saved Middlesex Panthers’ blushes with 50 not out that denied the Unicorns a famous Yorkshire Bank 40 win at Lord's.
Simpson, only recalled to the Middlesex side after Adam Rossington picked up a hand injury, kept a cool head to clinch a one-wicket victory with only five balls to spare.
The defeat was cruel on the Unicorns, who put in a spirited display with bat and ball only to stay bottom of Group C and without a win.
In pursuit of 209 for a fifth win in nine outings, Middlesex lost Dawid Malan to a good tumbling catch by keeper Lewis Hill, but re-grouped with a second-wicket stand of 65 between Paul Stirling and Joe Denly.
Stirling's fun ended for 42 when he danced down the pitch to Hampshire second XI leg-spinner Josh Poysden to be stumped by a couple of yards.
Poysden enjoyed further success pegging back Eoin Morgan's off stump as the Irishman looked to reverse sweep, and then in trapping Gareth Berg lbw as he attempted to slog sweep.
The Unicorns took pace off the ball by introducing left-arm spinner Luke Beaven at the Pavilion End who had Neil Dexter caught at deep cover off a miscued back-foot force.
Denly moved to a 63-ball fifty before he too fell to Beaven, bowled when working across the line to leg to make it 140 for six.
With nerves jangling, Ollie Rayner aimed to push drive at a Chris Skidmore away swinger to be caught behind, then Toby Roland-Jones went back in defence to his first ball to go leg before to Beaven, who finished with 3-43.
With Middlesex still needing 24, Skidmore found the outside edge of James Harris's bat for Hill to pull off a stunning catch diving full length in front of slip.
With four required, the Unicorns lost a concerted appeal when umpire Martin Saggers turned down their run-out calls as Simpson scampered a third to tie the scores.
The left-hander then clipped the first ball of the final over for a single to move to 50 from 48 balls to complete a nervy win.
Batting first after losing the toss, the Unicorns were thankful to veteran Keith Parsons, whose unbeaten 64 helped the visitors limp past 200 for a modest 40-over total of 208 for seven.
The part-timers looked to make a positive start as left-handed opener Matt Lineker clipped the second ball of the game from Steven Finn to the ropes in front of the grandstand.
Lineker, the former Derbyshire batsman, then played back in defence against James Harris only to drag an off cutter onto his leg stump.
Ex-Surrey bat Tom Lancefield also went for his shots, clattering five fours in a cameo 28 before a rush of blood led to his demise. Running down the pitch looking to drive at Roland-Jones, the left hander skied to cover where Malan clung on.
Michael O'Shea never found his form during a 42-minute stay that ended with a mistimed drive to mid-on against Roland-Jones.
The frenetic nature of the Unicorns innings continued through Hill, who clubbed 31 from 29 balls including four fours, before a slash drive against off-spinner Stirling flew into the hands of Berg at extra-cover to make it 103 for four.
Common sense prevailed when the experienced Garry Park and former Somerset stalwart Parsons combined for a fifth-wicket stand of 77 in 13 overs.
Park, who played 47 first-class games for Durham and Derbyshire, chipped in with 43 from 58 balls before skying to mid-on to give Roland-Jones figures of 3-33.
In the next over Aneurin Norman was caught behind off Finn for nought, leaving 40-year-old Parsons, with 130 first-class appearance to his name, to move to a 54-ball fifty with four fours and a six.
Beaven was caught in the deep off Harris in the 38th over but Parsons, playing in his 134th List A match, finished as unbeaten top-scorer.