Essex 257 for 7 (Westley 102, Dal 3-33) beat Derbyshire 248 for 9 (Came 97, Bennett 3-32) by nine runs
Tom Westley marked the 500th appearance of his professional career by hitting his 10th List A century in leading Essex to their first Metro Bank One-Day Cup victory of the season.
The Essex captain, who missed the opening-day defeat at Middlesex to be at the birth of his third child, played the pivotal role in the nine-run win with 102 from 97 balls.
Seamer Mackenzie Jones took a career-best 3 for 43 to hold up Derbyshire Falcon's rampant opening inspired by captain Harry Came's start-stop innings of 97 that threatened to take the game out of Essex's hands as they ultimately defended 258.
Put in on a green-top following a sharp pre-match shower, Essex batted largely with the handbrake on throughout as if the plan was to accelerate later on. They started by losing Luc Beckenstein to a diving catch at short midwicket by 42-year-old Wayne Madsen while Noah Thain was bowled, giving himself room and exposing his stumps to Anuj Dal.
By the time Matt Critchley joined him, Westley was beginning to find his stride. The pair gradually increased the tempo with a flurry of boundaries, Critchley pulling and driving effortlessly and Westley scoring mainly through midwicket. The partnership was worth fifty in eight overs, and 72 in a dozen, before Critchley was late on one from Dal and lost his off-stump.
Westley caressed his fifth boundary through midwicket to reach his half-century from 54 balls. However, after Critchley's departure he was unable to strike up a decent relationship with anyone else as Essex faded badly towards the end of their innings
Westley lost Grant Roelofsen to a leading edge to give Dal a return catch and best figures of 3 for 33. And the South African was followed by Simon Fernandes, who had just deposited Jack Morley over square leg for six, before driving low back to the spinner. Westley then put on a paltry 36 in six overs at the death with Zaman Akhter before holing out himself to long-on the ball after reaching three-figures courtesy of a 12th four.
The Falcons were quicker out of the blocks. Came's fifty took just 35 balls with 12 of them sent flashing to the boundary. Came took Derbyshire past 100 inside 15 overs by launching Critchley over long-on for six.
But with the first-wicket stand on 113, Martin Andersson skyed Mackenzie Jones to mid-on. Brooke Guest and Madsen lasted six balls each as a mini-collapse followed..
Mitchell Killeen has had to wait until late July for his Essex debut since signing last summer, but with his second ball had Dal beaten for pace.
The wickets put the brakes on underlined by the fact it took Came a further 59 balls to move his score to 97 before he fell lbw, playing down the wrong line to Jamie Porter. At that point Derbyshire required exactly 100 from 104 balls.
They were stalling when Jones claimed a third wicket with Yousef Bin Naeem palpably lbw offering no shot. The pressure mounted as Charlie Bennett returned to have Ross Whiteley caught at midwicket, Nick Potts at deep square leg in the space of four legitimate deliveries, and Ben Aitchison in the deep. With 15 needed from the last over, the task proved beyond the last pair.
