Essex 201 for 1 (Chopra 98*, Wheater 88) beat Glamorgan 200 (Cooke 59, Porter 4-29, Coles 3-41) by nine wickets
Varun Chopra and Adam Wheater put on a swashbuckling stand of 189 to inflict Glamorgan's fifth defeat in five Royal London Cup games this season and put Essex back into contention to move through to the knockout stage.
Until Wheater was stumped for 88 off 99 balls, it had looked like a race to see who would reach their century first. However, Chopra was left stranded on 98 when Essex passed the target of 201 with 18 and a half overs to spare.
Chopra's 84-ball innings, with nine fours and four sixes, took his season's tally to 445 runs from five innings.
Essex's victory was set up by some accurate and parsimonious bowling, led by Jamie Porter, and supported by Matt Coles's 3 for 41 in his first one-day outing since moving from Kent.
Porter, who had been rested for the last three games, bowled his 10 overs straight through to record best List A figures of 4 for 27.
By reaching their target so quickly, Essex improved their run-rate significantly, which could prove crucial if they win their remaining group matches against Sussex on Sunday and Kent next Wednesday and qualification goes to the wire.
For Glamorgan, Chris Cooke marked his 32nd birthday with sixes off successive balls in a 75-ball 59, and shared a 64-run seventh-wicket stand in 12 overs with Andrew Salter that lifted the general lethargy surrounding the Glamorgan innings.
Glamorgan, put in on a blameless wicket posted just 34 from the first 10 overs for the loss of their openers. Nick Selman epitomised the delusory pace with 10 from 22 balls before he nicked Porter low to Chopra at first slip. Aneurin Donald had already gone to the first ball he faced, edging a straight one behind in Porter's second over.
Ingram should have followed when he had 4 to another edge off Porter, but Chopra dived to his left, getting both hands to the ball but unable to hang on.
However, the first time Porter went around the wicket he ended Ingram's sequence of five successive one-day centuries against Essex. The left-hander was pinned on his crease as the ball slanted in, and he was out for 13.
David Lloyd did not last long, driving Wagner forcefully square to cover point where Simon Harmer took a spectacular catch. Connor Brown, who had managed four fours in a debut 31, departed to a one-handed catch by Adam Wheater in front of first slip to give Porter his fourth wicket.
Cooke and Wagg put on 42 for the sixth wicket in 16 overs before Wagg played all around one from Ravi Bopara and was bowled. Andrew Salter helped Cooke increase the rate and lifted Wagner over mid-on to put up Glamorgan's 150 as late as the 43rd over.
Salter hammered a second six over cow corner to dent Coles's otherwise parsimonious figures, but when he went for another next ball, he was caught on the boundary. He had hit 43 from 47 balls, and shared in a 64-run stand with Cooke in 12 overs.
Cooke passed fifty from 72 balls with a six flicked off his legs during the most expensive over of the innings which cost 17 runs. Bopara thought he had Cooke, on 59, caught off a head-high no-ball by Cook, but the South African swung at the next ball and Bopara took the return catch above his head.
Coles wrapped up the innings with wickets in successive balls for figures of 3 for 41.
Chopra showed the Essex intent. Ruaidhru Smith was twice deposited over the leg-side boundary ropes for six in his first two overs.
With Wheater finding the gaps, the opening pair had 50 on the board inside eight overs, 100 up in the 16th. The wicketkeeper was first to his personal fifty from his 55th ball.
Chopra's half-century came from 45 balls and contained five fours and those two sixes.
When he was 63, Wheater had a reprieve as a lofted drive flew through van der Gugten's hands at midwicket. The 150 partnership came up in 26 overs before Chopra clouted Salter for a straight six, and then helped himself to four and another six in successive balls off the spinner.