Match scores (The Oval)
Essex won by one wicket
Surrey inns: (47.4/50) 210 (Foakes 77*, Borthwick 37, T Curran 33; Quinn 3-34, Bopara 3-46).
Essex inns (49.2/50) 212-9 (Ten Doeschate 45, Harmer 44*, Bopara 36, Westley 34; Dernbacj 4-31, Batty 2-45, S Curran 2-46
Match report
Tail-ender Simon Harmer hit an unbeaten 44, including the winning boundary, to steer Essex to a thrilling one-wicket win over Surrey in a low-scoring Royal London One Day Cup clash.
Harmer, who has five Test caps for South Africa, saw his side past their target of 211 in the final over at the Kia Oval to inflict a second defeat on last season's beaten finalists.
Surrey were indebted to Ben Foakes for an unbeaten 77 - the only half-century of the match - as they posted a modest 210 before being dismissed with 14 balls unused.
Mark Stoneman carved Matthew Quinn to point and Kumar Sangakkara was caught behind off a lifter from Wagner to leave Surrey 24 for two before Essex skipper Ryan ten Doeschate introduced off-spinner Harmer at the Pavilion End and enjoyed immediate rewards.
Dominic Sibley pushed inside an arm ball and was stumped. Then, five runs later, Rory Burns rushed down the pitch aiming to drive, only to see the ball turn past his outside edge and peg back off stump to make it 46 for four.
Scott Borthwick and Foakes dug in to add 73 before Borthwick, having previously top-edged short of the keeper and chipped close to long-on, dragged on against Ravi Bopara to depart for 37.
Sam Curran edged Bopara's next delivery behind for Adam Wheater to pull off a stunning low catch in his left glove, with Curran's brother Tom surviving the hat-trick ball.
Foakes, stylish and untroubled throughout, reached his 50 from 69 balls and went on to add 57 for the seventh wicket before Tom Curran (33) nicked an attempted pull against Quinn (three for 34) through to the keeper.
Gareth Batty and Jade Dernbach holed out to long-on while Bopara (three for 46) bowled Ravi Rampaul as Surrey limped past 200.
The Eagles slipped to 12 for three inside seven overs as Nick Browne wafted outside off against Sam Curran to be caught by a tumbling Foakes, Alastair Cook was pinned on the back foot by the same bowler and Varun Chopra feathered to slip off Dernbach.
Under leaden skies and with the floodlights on, the visitors rebuilt through Tom Westley and Bopara's stand of 66 that ended when Westley (34) top-edged a Dernbach bouncer to mid-wicket.
Bopara (36) drove without moving his feet against Dernbach to gift Foakes another catch, while Wheater chopped on against Rampaul to leave Surrey scenting blood at 112 for six.
Ashar Zaidi suffered a rush of blood to the head by dancing down the pitch against Batty only to chip to mid-off but Ten Doeschate (45) showed all his experience to add a crucial 51 with Harmer before edging a leg-glance against Batty into the gloves of Foakes.
Harmer and Wagner came together with 32 required, a deficit they halved before Dernbach, with another fierce, throat-high bouncer, had Wagner caught on the hook at deep square to finish with four for 31.
With four required off Sam Curran's final over, Harmer sealed it by sweeping a low full-toss to the backward square boundary with four balls to spare.
He finished unbeaten on 44, his best in List A cricket as Essex landed their second straight win.