Essex 262 and 13 for 0 lead Surrey 187 (Jacks 70, Harmer 6-67, Porter 4-53) by 88 runs
Simon Harmer bagged his 18th first-class five-wicket haul for Essex as the hosts helped themselves to a healthy first-innings lead over Surrey in the Bob Willis Trophy. Offspinner Harmer has now taken 229 wickets since joining Essex at the beginning of the 2017 season as he returned figures of 6 for 67.
Harmer was the County Championship's leading wicket-taker with 83 scalps last year and looks likely to replicate that record in the Bob Willis Trophy - with no rival taking more than his 12 poles so far in the competition.
The South African shared the wickets with trusty partner Jamie Porter, who claimed 4 for 53, as Surrey were bowled out for 187, in response to Essex's 262 - a deficit of 75, which rose to 88 in a wicketless four-over twilight burst.
After Essex had lost their last three first-innings wickets inside seven overs, Porter blasted a hole at the top of the Surrey batting order with two wickets in two balls. The seamer forced Ryan Patel to hand Harmer a regulation catch at second slip with his sixth delivery, before Scott Borthwick tucked off his hip to Feroze Khushi at midwicket.
Will Jacks saw off the hat-trick as he dug out a yorker as he began to work his way towards a well-made 70. At the other end, Mark Stoneman struggled to get out of neutral gear as he scored five singles in 65 balls before edging Harmer to Alastair Cook, before Jamie Smith had the top of his off peg knocked back by Porter.
Jacks was dropped on 26, 31 and 46 - twice by Varun Chopra at short leg and once by Aaron Beard at deep fine leg - but moved to his seventh first-class fifty in 81 balls. The 21-year-old made batting look uncomplicated, on a sun-kissed pitch best suited to patience, with 12 boundaries struck all around the Cloudfm County Ground, Chelmsford.
He had been joined by Laurie Evans - who was playing his first innings in a decade for Surrey - for a valuable 80-run stand for the fifth wicket. Evans, on loan from Sussex due to Surrey's 14-man absentee list, had played four first-class matches during his first stint at the county between 2005 and 2010 before joining Warwickshire.
He was the aggressor, particularly against Harmer, whom he struck for three perfectly nailed sweeps and a six over long-on.
Both fell either side of tea to spark a second collapse of the innings - Surrey losing three wickets for three runs as Harmer took control. Jacks pushed Harmer to midwicket before Porter bowled Evans.
Since Harmer arrived at Chelmsford on a Kolpak deal, he and Porter have shared 428 first-class wickets and two County Championship titles together.
Chopra made amends for his early spills when he clung onto a stunner under the lid to dismiss Gus Atkinson, and then held onto a loopier catch off Rikki Clarke's bat-pad. Harmer's five-for was confirmed when Adam Finch clipped around the corner to Tom Westley.
Essex were frustrated for 10 overs by the last-wicket pair of James Taylor and Amar Virdi, before the former picked out Nick Browne at cow corner. Browne immediately turned around to stick his pads on for four overs with Cook - the pair getting through Clarke and Virdi's overs unscathed.
Earlier, Essex had only added nine runs to their overnight score of 253. Clarke was the chief tail destroyer as he returned figures of 3 for 26 - only going for one boundary in his 21 overs.
Handed the new ball at the start of the day, Clarke taught a young Surrey bowling attack how to rip through a tail. The fresh leather caused extra bounce off a length, which caught Beard out as he edged behind in the third over of the day, before a fuller delivery pinned Sam Cook lbw.
Clarke's lesson was heeded by debutant Atkinson who wrapped up the innings when Harmer top-edged a pull shot to Patel at cover.