Essex 289 for 5 (Bopara 74*) beat Kent 285 for 8 (Coles 91, Quinn 3-50, Zaidi 3-33) by five wickets
Essex's captain Ravi Bopara batted himself back into form in white-ball cricket with a 78-ball 74 to take Essex to the top of the southern group in the Royal London Cup.
Essex reached their 286-run target with five balls to spare to remain unbeaten in this season's 50-over competition.
That Essex had a sufficiently challenging total to chase was largely down to Matt Coles and an eighth-wicket partnership of 140 in 14.2 overs with James Tredwell.
Coles savaged the Essex attack, bludgeoning 91 off 52 balls with 10 fours and five sixes after Kent, put in, had looked in serious trouble at 135 for 7, but his efforts proved to be to no avail.
Bopara had struggled with the bat, scoring just 49 runs in seven previous one-day innings this season.
It was not a swashbuckling knock by any means, with just one four to his account when he reached his half-century, but he kept the Essex reply on track with quickly-run singles and twos in fifty-plus partnerships with Dan Lawrence and Ryan ten Doeschate.
"I'd got out to run-outs a couple of times, so that's two innings where I might have got a lot of runs, who knows?," Bopara said. "Then you put yourself under pressure if you're not scoring, so it's nice to get a few today. It was good as well ahead of the T20 tomorrow where we've got to be positive and hit the ball hard.
"We consider ourselves as a chasing side - our batting is a lot stronger than our bowling. When you're chasing you're always going to be under pressure, you're looking at the scoreboard and it jumps up from seven to nine and a half an over, but the idea is never to panic. As long as you keep your wickets in hand you've always got a very, very good chance."
Coles was disappointed that his efforts did not secure victory. "I think we should have been able to defend that total," he said. "We knew it was going to be tight. Cowdrey bowled very well for us, and so did Tredwell, and I thought we started very well. But we ended up giving away 23 extras and that's not brilliant."
Kent had passed 50 without alarms but Essex's bowling breakthroughs began when Matt Quinn switched to the Hayes Close End.
Joe Denly was beaten by movement off the pitch and was bowled for 28. Daniel Bell-Drummond had been becalmed after an early onslaught and departed in the 18th over for 28 when he played on to Quinn. And Quinn had his third wicket in his next over when he nipped one back to send Sam Northeast's middle-stump cartwheeling.
Ashar Zaidi's first ball of his second spell accounted for Fabian Cowdrey, who chased a wide ball down legside and was stumped after managing just four runs from 22 balls.
Sam Billings had obviously not paid attention as four balls later he fell in similar fashion. Zaidi's delivery was so wide of off-stump it barely pitched on the cut strip, Billings fell and lost his bat in trying to reach back into the crease. James Foster dived to remove the bails.
When Zaidi did bowl straight, Darren Stevens twice sent the ball soaring over the Pearce Stand towards the River Can for sixes.
Alex Blake was sixth man out when he looked for a quick single as he pushed Zaidi to Jesse Ryder at square leg, only to slip as he tried to get back. Stevens went next as he tried for a third six off Zaidi, but was well held on the midwicket boundary by Nick Browne. At 135 for 7, Kent were in deep trouble.
But Coles and Tredwell turned the visitors' innings around and added fifty in 7.1 overs, with Coles hitting both Ryan ten Doeschate and Ravi Bopara straight for six.
Bopara was particularly savaged, going for 35 off two overs before Coles reached his own half-century off 40 balls when he drove Quinn through the covers for two.
Coles brought up the century partnership in exactly 12 overs with an uppish sweep off Bopara for four and then hoisted his third straight six next ball.
Ten Doeschate was hit for successive sixes - over mid-off and midwicket - followed by a four before Coles was bowled going for another heave to depart for 91.
Tredwell had been the silent partner in the partnership and finished 47 not out off 42 balls with six fours.
In reply, Essex lost Tom Westley in the fifth over when he got a leading edge to steer Stevens to Denly at mid-off before Browne and Ryder settled into a largely serene period.
Browne punished Stevens with a lob over the infield for four followed immediately by a six to long leg. The fifty partnership came up in 9.2 overs with Ryder slamming a full-toss from Ivan Thomas through midwicket for four followed by another uppishly through the covers.
Ryder reached his individual fifty from 61 balls with a single off Tredwell, but popped the next delivery he faced into Stevens's hands at backward point. He had helped put on 82 in 17.2 overs for the second wicket. Browne went in Tredwell's next over, victim of a smart stumping by Billings for 49 from 60 balls.
Tredwell, though, was punished by 18-year-old Dan Lawrence, who hoisted two successive straight sixes as he and Bopara clocked up a fifty-run partnership in 9.3 overs. Lawrence eventually went on 35, caught at deep extra cover by a tumbling Thomas off Cowdrey to end a 64-run partnership.
Bopara reached a painstaking 50 off 64 balls with a solitary, straight-driven off Cowdrey early on. There were ironic cheers when he hit his second boundary, driving Coles through midwicket to move from 52 to 56.
The fifth-wicket had just put on a century in 12 overs when ten Doeschate, attempting to win the game with one hit, holed out to Cowdrey on the extra cover boundary for 45.