Despite an exceptional spell of left-arm fast bowling by Reece Topley, Surrey suffered their second defeat of the Vitality Blast this season at the hands of the Essex Eagles.
Returning from England duty to face his former club, Topley struck early in the powerplay to remove Adam Rossington before taking three wickets at the death, including top scorer Paul Walter, finishing with figures of four for 37.
Michael Pepper boosted his Chelmsford tally to 304 runs in six innings – and 386 in total – with 48, as Walter continued his fine form with 49 to help the Eagles post 198.
Aaron Beard then returned career-best figures, on just his second appearance of the season, removing openers Will Jacks and Jason Roy to eventually pick up four for 29 – as Surrey fell 43 runs short.
Surrey remain top of the South Group and will host a home knockout game – Essex boosted their hopes of securing a home quarter-final to move to seven wins out of 12.
On a fixture celebrating Essex Pride, Surrey stuck the Eagles in to bat and had Rossington caught behind in the second over, with the first two overs only going for six before Pepper found his rhythm.
He struck 30 from the first nine balls he faced, dealing only in boundaries bar a dot and a single. The South African-born batter began by scooping a six, something he replicated in the following over between a cut shot and followed by four consecutive boundaries off Dan Moriarty.
His hitting paused after a strike to the box, which he never really recovered from as he limped to five more runs before he reversed straight to third – the second wicket in three balls for Sunil Narine.
Essex rebuilt and their second 10 overs were well paced – with only seven dot balls, along with four wickets coming in the second half of the innings for 110 runs.
Walter was the chief architect of the scoring with his 49, which included four sixes, as he put on 51 with Dan Lawrence and 44 with Simon Harmer.
Topley made sure his former county didn’t get to 200 as he chipped away throughout and added Walter, Daniel Sams and Harmer to his earlier Rossington dismissal to end up with four for 35.
Beard made an electrifying start as he picked up both Roy and Jacks – the ball after selecting a new bat – brilliantly caught at short fine leg by Tom Westley.
Jamie Smith and Rory Burns put on a calculated 57, while still keeping up with the rate before Lawrence stuck one through Smith. Burns followed by striking Beard to mid-on and Narine fired back at Harmer as Surrey began to lose wickets regularly.
Laurie Evans fell soon after when sub fielder Ben Allison produced a spectacular solo relay catch at wide long-on – Harmer grabbing two wickets in the over to return two for 20. Beard had Tom Lawes, who made his T20 debut for Surrey, caught at mid-on for his fourth.
Sams pilfered Chris Jordan and Sam Cook had Conor McKerr as Surrey reached 155 for nine.