Essex 275 for 5 (Ryder 100, ten Doeschate 48*) beat Sussex 271 (Joyce 73, Wright 50, Taylor 50, Quinn 4-71) by five wickets
Jesse Ryder scored his ninth List A hundred as Essex pulled off a successful run chase for the second successive day to make it two wins out of two in the Royal London One-Day Cup.
Having overhauled a target of 310 to beat Hampshire on Sunday when Ryder made 71, they beat South Group rivals Sussex by five wickets with six balls to spare under the Hove floodlights after being set 272 to win.
Sussex were still left without a List A win since August 2014.
Ryder accelerated after scoring his first 50 off a relatively sedate 71 balls, reaching his hundred from a further 38 deliveries with six fours and three sixes.
With nine overs left and 80 needed the odds favoured Sussex, but Ryder and Ryan Ten Doeschate plundered 83 in 9.3 overs and when Ryder was yorked by Chris Jordan for 100 Ten Doeschate, who finished unbeaten on 48, got the job done, hitting Ajmal Shahzad for four and six at the start of the penultimate over before former Sussex player Ashar Zaidi struckk successive boundaries to seal an impressive victory.
Left-armer George Garton and leg spinner Will Beer were the pick of the Sussex attack. Garton, 19, impressed on his List A debut. He took a simple return catch off a leading edge from Tom Westley (32) and had Dan Lawrence (25) caught at long on while Beer enjoyed a pitch where the odd ball gripped and turned. Nick Browne (44) was expertly stumped by Ben Brown after he was beaten by a ball that spun past his outside edge and Essex captain Ravi Bopara (12) was caught at short third-man off a top edged sweep.
Earlier, three Sussex batsmen made half-centuries but neither Ed Joyce (73), Luke Wright (50) or Ross Taylor (50) were able to go on and play the sort of innings Ryder would produce on a pitch which played well despite being used for the third time in five days.
Joyce once again looked in superb touch, sharing a stand of 108 with skipper Wright for the second wicket during which Wright passed 4,000 List A runs and Joyce brought up his 50 and Sussex's 100 with a six over mid-wicket off Matt Dixon. Wright's half-century came off 58 deliveries (3 fours) but two balls later he was stumped off a wide outside off stump by Zaidi.
It was the first of three wickets to fall in four overs. Joyce mis-timed a drive to wide mid-on and Matt Machan was bowled by Zaidi trying to reverse-sweep his third ball.
Taylor was put down by Ryder at backward point on five and made the most of his reprieve, putting on 59 in 12 overs with Brown.
The former New Zealand skipper scored 50 off 58 balls (4 fours) only to hole out to long on and it needed a cameo from Jordan to take Sussex to a competitive total. The England international cleared the ropes three times in his 36 off 22 balls before becoming one of four victims for Matt Quinn.