Glamorgan 281 for 7 (Ingram 142, Bragg 59, Wagner 4-58) Essex 280 for 9 (Chopra 124, Bopara 56) by one run
Essex missed the chance to maintain their 100% record in the competition when they lost in an exciting last over finish against Glamorgan who gained their second win from five games.
Glamorgan were indebted to Colin Ingram, who scored a superb 142, which included four fours and eight sixes, two striking the pavilion roof, while Varun Chopra, who made 124, threatened to win the game for Essex. Following his dismissal, panic set in and they were two runs short of the seven they required to win from the last over.
Glamorgan, who had elected to bat on the pitch used for the previous one day game on Friday, soon lost their openers, Jacques Rudolph and David Lloyd, who were both dismissed by Neil Wagner. Ingram and Will Bragg then played with extreme caution against Wagner and James Porter and after the first 10-over Powerplay the score had only progressed to 26 for 2.
Their caution, however, was rewarded as the third-wicket pair put on 98 in 22 overs, before Bragg, having scored 59 from 79 balls, was stumped down the leg side from Ashar Zaidi's left arm spin - the second time in successive innings for Bragg to be dismissed in this way.
Kiran Carlson then joined Ingram, and the 18-year-old, who top scored in the previous game, again showed plenty of promise with an array of attacking strokes, scoring 36 from 33 balls, before giving Ravi Bopara a return catch.
Ingram, playing every ball on merit, reached his 16th one-day hundred, and his third against Essex, before launching a furious assault on the attack, notably Ryan ten Doeschate, who was struck for three sixes in an over and with 87 runs scored in the last ten overs, Essex faced a challenging target of 282.
Glamorgan's opening attack struck immediately. When Marchant de Lange had Nick Browne caught behind and Tom Westley was bowled by Michael Hogan, Essex were 2 for 2 after nine balls.
The visitors then replicated Glamorgan's recovery, as Alastair Cook and Chopra settled into a productive partnership. De Lange didn't help his, or Glamorgan's cause, by greeting Chopra with three short pitched deliveries that were struck for sixes over the midwicket boundary. He outscored Cook, reaching 50 from 59 balls, and they added 103, before Cook gave a low catch to cover point.
Ravi Bopara then supported Chopra in a stand of109, with Chopra reaching his third List A century for Essex, and Bopara an attacking 56, before Bopara was unluckily run out, when Craig Meschede dropped a return catch from Chopra, only to deflect the ball on to the stumps at the bowler's end.
Essex then faltered, losing Adam Wheater and Chopra, who was bowled by Meschede, and at 239 for 6, the visitors required 43 from the last six overs.
Essex were favourites at that stage, but when Ashar Zaidi and ten Doeschate were dismissed, and Simon Harmer ran himself out, four runs were needed from the final ball with the last pair at the crease. They could only manage two and Glamorgan had won by the slender margin of one run.