Essex 262 (Khushi 66, Wheater 52) and 261 (Virdi 4-85, Finch 4-38) beat Surrey 187 (Jacks 70, Porter 4-53, Harmer 6-67) and 167 (Harmer 8-64) by 169 runs
spun his way to match figures of 14 for 131 as county champions Essex beat Surrey by 169 runs to maintain their perfect start to the Bob Willis Trophy.
Offspinner Harmer had taken 6 for 67 in the first innings, before producing his well-worn fourth-innings trick by steamrolling Surrey with 8 for 64. Harmer had been on course for the first 10-wicket haul in county cricket since Ottis Gibson for Durham in 2007, but Aaron Beard
took two late poles to deny him history.
Incredibly, Harmer, who bowled unchanged from his River End on day four, is playing through an abductor strain, which medical staff had recommended would rule him out for up to six weeks.
Essex, who have now won their last ten first-class matches at Chelmsford, jump to the top of the South Group table, while heavily depleted Surrey remain the only winless side in the conference.
"The wicket at Chelmsford always suits me," Harmer laughed. "Professional cricket is about performance and failures and when it is your day you have to make the best of it because I don't know what will happen next week at Hove.
"When the wicket is turning, I'm going to cash in as much as I can and that's what I did today. I knew they would be under pressure. I am very happy with the way the ball is coming out at the moment but cricket is a funny game so you can't take anything for granted, but touch wood may the form continue.
"When I got to six wickets the boys started talking about how if I pulled my finger out then I might have been able to get all 10. Those things don't happen very often in cricket but I'll take that catch and an eight-for any day."
Harmer had started his haul with the last ball of the third day, when Mark Stoneman was leg-before to one which didn't turn - with Surrey requiring an unlikely 310 to win from the final day.
It took his 23 morning deliveries to breakthrough Scott Borthwick's defences as the once-capped England Test player attempted a forward defence only to see the ball spin off his outside edge and into Adam Wheater's gloves.
Surrey weren't going to roll over and hand Harmer and Essex the wickets they required, with Ryan Patel and Will Jacks providing a blocking rearguard.
Patel, in particular, seemed immovable as he grappled to hold onto his wicket, showing intense concentration to see out 97 balls. But the 22-year-old lapsed for a moment to sky a pulled short to Jamie Porter at midwicket while attempting to cash in on a shorter delivery from Harmer.
Jacks saw out 93 balls with his laser focus, while also putting on a 51-run stand with Jamie Smith
- as the pair held Essex up for 20 overs.
But having refuelled over lunch, Harmer took his next two wickets in three balls. Jacks, who had batted beautifully scored 70 in the first innings, attempted to go back and work to leg side only to be pinned - that brought up Harmer's 10 wickets in the match. Two balls later, Evans, on loan from Sussex, skipped down the track, missed the ball and was stumped by Wheater.
Rikki Clarke twice used his feet to strike Harmer for two boundaries down the ground but departed for 14 from 27 balls when Varun Chopra held on to a simple catch at short leg.
Wicketkeeper-batsman Smith once again showed why he is so highly rated at The Oval with a two-hour vigil. The 20-year-old scored 45 in 89 balls but departed at the hands of Harmer - although not through his bowling. Smith edged Beard to second slip, where Harmer swooped low to pull off a stunning catch, which ended his own shot at immortality.
Harmer returned to his first skill again in the next over as Gus Atkinson, who had hit him for six, ran past a straight one to be bowled.
Beard saw off James Taylor, caught at cover, for a seven-ball duck before Harmer wrapped up the victory at 3.35pm when Amar Virdi slapped to mid-off.