Laurie Evans hit a magnificent unbeaten 88 from 52 deliveries as Surrey pulled off a thrilling last ball victory at Chelmsford.
They did so when Gus Atkinson found the boundary from the final delivery after Essex had posted a total of 195 for eight.
Evans struck four sixes and seven fours in his 52-ball effort after Cameron Delport and Adam Wheater both scored half-centuries to give Essex a great start to their innings.
The openers put on 105 but in the end it did not prove enough once Evans got into his stride to point the way to a magnificent Surrey victory.
The Essex openers started off like an express train as they raised the 50 in the fourth over. Wheater collected three successive fours at the expensive of Gus Atkinson during an over in which Delport struck a six. Delport then punished Jamie Overton for three boundaries in succession and as the runs continued to flow the total moved into three figures by the ninth over.
Surrey were to claim their first wicket with the total on 105 when Daniel Moriarty had Delport taken at fine leg by Hashim Amla to end an innings of 64 that contained eight fours and six sixes and came from 32 balls.
At the half-way stage Essex had reached 117 but then wickets started to fall in quick succession. Daniel Lawrence went for 17 when he was caught at deep cover by Overton to provide Moriarty with another success and the slow left arm spinner struck again to end the fireworks provided by Wheater.
He perished when he was easily stumped by Ben Foakes after contribution of 63 from 40 deliveries with the help of five fours and three sixes.
Off-spinner Gareth Batty then removed Paul Walter and Simon Harmer with successive deliveries while Topley claimed three quick wickets by removing Michael Pepper, Jack Plom and Aaron Beard. Following their great start Essex will have been disappointed at not reaching the 200 barrier.
Ryan ten Doeschate finished unbeaten with 27 from 20 balls while Moriarty and Reece Topley finished with three wickets each at a cost of 36 and 38 respectively. Batty proved the most economical with two for 23 from his allocation of four overs.
Surrey’s response began disastrously with Amla departing lbw to Aron Nijjar to the spinner’s first ball of the innings.
That early setback, however, did not stop Will Jacks and Evans moving into top gear as they brought up the 50 in the fifth over of the innings.
But then Surrey lost two quick wickets. Jacks was bowled by Sam Cook for 29 that contained six boundaries while former Essex player Ben Foakes was caught by Walter to provide Beard with success.
Evans continued to inflict punishment with some powerful hitting and reached a fine half century from 27 balls with the help of five fours and two sixes.
Soon afterwards he dispatched Harmer and Lawrence for sixes but lost Rory Burns for 30 after the pair had shared a partnership of 78 from just under eight overs.
He was caught on the long-off boundary by Nijjar with a total on 137. With four overs left Surrey required 51 but were to lose Jamie Overton and Jamie Smith in the same Aaron Beard over, at which stage the visitors were still 21 runs short of their objective with less than two overs remaining.
But in the end they squeezed home to register a victory that virtually put paid to their opponents making further headway in a competition they won last season.