Essex beat Middlesex by three runs in one of the most exciting finishes witnessed at the Cloud County Ground for some time.
Half centuries from captain Tom Westley and a maiden fifty from rookie Charlie Allison had seen the hosts post 298-9.
But the visitors looked to be cruising as their top order all contributed, with Mark Stoneman going close to a century.
Then having reached 236-3, the wheels came off in stunning fashion as Essex rallied to claim a memorable victory.
Something had to give, with neither side yet to taste victory in the competition in a combined total of five previous outings.
Essex, rained off at Lancashire, thrashed by Nottinghamshire and eclipsed at Hampshire, had Michael Pepper back from The Hundred, having not played in successive matches for London Spirit.
And he hit Ishaan Kaushal for a trio of boundaries in the second over, two of them from full tosses, to get the scoreboard moving along early.
Ethan Bamber was then hit nicely down the ground by Pepper, who pushed Kaushal away square on the off side for his fifth boundary, then launched him over extra cover for the first six of the day.
Das opened his account with a lovely push off Bamber through point, before Josh de Caires came into the attack and saw his first ball pulled behind square by Pepper, but his next hit to mid-off Martin Andersson, who took a good catch over his shoulder, to fall for 31 off 18 balls.
Das took up the mantle in Bamber's next over, hitting successive boundaries - through mid-on and beautifully down the ground - but then picked out Joe Cracknell coming in from the square-leg boundary to give De Caires his second scalp.
And the off-spinner struck again two balls later, beating Beau Webster through the gate and seeing John Simpson complete a smart stumping to leave the hosts 56-3.
Luc Benkenstein, back in the side as a batter after recovering from a stress fracture of the back, and captain Westley each found the rope as the hosts ended the powerplay on 67-3.
Leg-spinner Luke Hollman came into the attack in place of former North Middlesex teammate Bamber at the River End and the Essex duo seemed happy to milk the bowling, before Benkenstein clubbed De Caires over long-on for six and Westley guided a nice dab to fine third man to make it 93-3 after 14 overs.
Benkenstein played a very neat on-drive off Martin Andersson to put Essex into three figures, with Westley cutting the same bowler away to the rope in his next over to bring up the 50-run stand off 58 balls.
But Benkenstein fell for 29, edging Hollman to Simpson, with the Essex total on 111, and number six Simon Harmer was trapped leg before by Hollman not long after in only the second maiden over of the innings to that point.
De Caires returned at the Hayes Close End, but saw his first delivery swept away for four by Charlie Allison and was pulled into the leg side for another boundary by the youngster in the 27th over.
Westley moved to within one of his half-century with a glorious cover drive off Ryan Higgins, cutting the next ball for a single to reach 50 off 59 deliveries.
And the sixth-wicket pair brought up their half-century stand off 55 balls, before Sam Robson came into the attack for the visitors.
Robson found a way through Westley's defences in his fourth over, bowling the Essex skipper for 80 off 90 balls to leave the hosts 210-6.
But Will Buttleman proved a useful foil to rookie Allison and struck Robson for a big straight six into a surrounding garden.
And young Allison followed suit a few balls later to bring up his maiden half-century from 59 balls, then carted the returning Bamber over Cow Corner and into the River Can, most likely.
The 50-run stand for the seventh wicket came up off just 47 balls, which Allison celebrated by smashing Higgins away for six, before Buttleman pulled Bamber over the square leg boundary, just.
Allison edged Bamber into his own stumps to fall for a hugely entertaining 70 off 65 balls, though, and Buttleman holed out to long-on off Kaushal.
And Aron Nijjar was run out by Bamber attempting a very risky single, after Aaron Beard had clubbed Kaushal for six, as Eagles closed on 298-9
Stoneman got the Middlesex run chase underway with a four square of the wicket off Beard, then managed another off the same bowler, off an inside edge, in the third over, followed by a superbly timed clip away through mid-wicket and a six over fine leg.
Left-arm spinner Nijjar replaced Beard from the River End and began well, conceding just two singles, but Stoneman clipped Porter over square leg for six - after a failed attempt at a ramp shot the ball before.
And Cracknell pulled Nijjar away for successive boundaries to bring up the half-century opening stand off 38 balls.
Harmer relieved Porter after three overs, but was lofted over cover by Cracknell for four, then pulled past Webster at square leg for another boundary.
And he saw a steepling chance off Stoneman put down by a diving Benkenstein in the deep as Middlesex ended their powerplay on 76-0.
Webster came into the attack and alternated between bowling seam to Cracknell and spin to Stoneman, but the batters remained on top.
Left-hander Stoneman brought up his 50 off just 39 balls, then left Harmer with his hands on his head after playing and missing later in the 14th over.
Pepper was next to feel frustrated after failing to take his chance to run out Stoneman, who subsequently paddled Nijjar behind square for four to bring up the hundred partnership in 89 balls.
And Westley brought himself into the attack after five overs from Harmer had not been able to make the breakthrough, with Cracknell spooning the ball between three converging fielders and then diving to make his ground when attempting a second run.
There was no escape for Cracknell, though, when he hit Nijjar to a diving Das in the next over to fall two short of his half-century, having faced 54 balls in a 124-run stand.
Robson got off the mark with the first three of the match - in its 69th over - then pushed Westley effortlessly through cover for a boundary.
And Stoneman produced another inch-perfect cut between Nijjar and Allison, whose dive to prevent the ball reaching the rope proved in vain.
Harmer returned in place of Westley, but Robson dabbed him past Porter for four as Middlesex brought up their 150 in the 22nd over.
And Robson had a let-off when dropped by Das, just above the turf, at backward point then stood firm when Buttleman claimed an edge off Webster, who hurled the ball to the ground in frustration, as the visitors reached the mid-point of their innings ahead of the run rate at 166-1.
There appeared to be an inevitability about the run chase at this stage, before Beard then bowled Robson for 26 to rouse the home fans.
But Jack Davies then helped bring up the 200 in the 32nd over, before Stoneman - having played out the first maiden of the innings to Westley - hit Beard to Jamie Porter for a great catch on the boundary to fall for 93 off 97 balls.
Davies then reverse swept a pair of boundaries in successive overs from Nijjar, who was hit into the upper tier of the members stand twice by Ryan Higgins in an over that yielded 21 in total.
But Harmer finally got some reward when bowling Davies for 32, before Webster had Higgins dragging on, when giving him the charge, to make it 246-5.
Hollman was then run out without facing a ball, after a mix-up with John Simpson over a second run, as Das hurled the ball to Buttleman.
That made it 247-6 and had Essex fans believing in the impossible, although Simpson pulled Webster away for a boundary.
Porter then returned to the attack and bowled De Caires all ends up to leave it very much on a knife edge, with Webster and Nijjar helping to turn the screw as the dot balls began to add up.
The target was 33 off 42 balls, before Simpson hit Porter for successive boundaries at the start of the 44th over to ease Middlesex nerves.
And a deliberate Simpson edge off Nijjar for four left the visitors needing 17 from 30 balls, before the left-hander swept Westley to the boundary to leave his side needing just 11 from 24.
Simpson had a reprieve when drilling the ball back to the off-spin of Webster, who could not hold onto a tough chance, but the Australian bowled Andersson with the last ball of the 47th over, with the target now in single figures.
And Westley then bowled dangerman Simpson with Middlesex seven runs short of their goal to the delight of the home support.
It left the last pair of Bamber and Kaushal needing to score seven off 12 and the senior partner managed two when pushing Webster down the ground.
But having edged the fifth ball of the penultimate over down to the boundary and turned for a second run, after a superb piece of fielding by Allison, he was run out to leave Essex as the winners.
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