Leicestershire 243 (Robson 74) and 159 for 6 (Robson 56, Cosgrove 38) beat Essex 268 (Bopara 61, Westley 57) and 133 (McKay 4-37) by four wickets
Leicestershire inflicted Essex's first defeat in the Championship with only 11 balls to spare as they chalked up their second Specsavers County Championship win of the season.
In favourable conditions during the afternoon, they bowled out Essex for 133 before chasing down the 159 they needed in a rapid evening run chase.
Chelmsford has been a happy hunting ground for Leicestershire in the last two summers: last year they ended a run of 37 Championship matches without a win with a six-wicket victory.
Angus Robson hit his second half-century of the match, finally falling 25 runs short of the target for a 75-ball 56. He was ably assisted in a third-wicket stand of 63 in 7.1 overs with Leicestershire captain Mark Cosgrove
In a nicely paced innings, Leicestershire accelerated when the last hour was announced, at which point they had required 88 off the statuary 16 final overs with eight wickets in hand. There was still time for a late wobble with three wickets going down for nine runs with victory in sight.
The day had begun in heavy drizzle and ended in bright sunshine. In between Leicestershire made the most of the overcast conditions, their seamers swinging the ball every which way.
Clint McKay weighed in with 4 for 37 in the second innings for match figures of 8 for 84 to take his season's tally in the Championship to 31. At times he was unplayable.
He was ably supported by Charlie Shreck, Ben Raine and Rob Taylor, who shared the other wickets equally between themselves, and who all revelled in the heavy cloud cover.
Essex lost three wickets in a morning session shorn of 40 minutes and 10 overs because of bad light. The nightwatchman Matt Quinn hung around for six overs until his 43-ball stay was ended on 10 when he hooked backward of square to Raine.
Jaik Mickleburgh followed in the next over when he gloved Raine down the legside and Ned Eckersley pulled off an incredible catch. The wicketkeeper raced 15 yards to his left before diving to catch the ball one-handed close to where leg gully would be.
Eckersley took a more routine catch behind the stumps when Ravi Bopara ended a 26-ball two by edging a lifting delivery from Charlie Shreck. Third ball after the break, they lost the man who was taking the fight to Leicestershire, with Tom Westley pegged lbw by Rob Taylor for 25.
Jesse Ryder had survived a caught-and-bowled chance to Taylor before he played no shot to a ball from McKay that pitched outside off-stump and took the top of middle.
McKay claimed his fourth wicket of the innings when he had Ryan ten Doeschate caught at point by Neil Dexter before Dan Lawrence edged Raine to Robson at first slip for 22 from 48 balls.
James Foster and David Masters shut up shop either side of tea, scoring just six runs in 10 overs, before Shreck brought one back to bowl Foster, who offered no stroke. Masters followed without addition to the tea score, trapped in his crease by Taylor for four.
When Leicestershire's run chase began, Essex might have made an early breakthrough when Robson was seven. He pulled Porter straight into the usually safe hands of Bopara at square leg; the ball went in and immediately came out again.
Robson had another escape nine runs later when he edged Masters high to first slip where Ryder got a hand to it, but couldn't hold on.
The introduction of Bopara in the 12th over heralded the first Leicestershire wicket, ten Doeschate running around from mid-on to clutch a hoick from Paul Horton.
The visitors showed their intent by promoting Taylor to No 3, but he lasted just seven balls when he was beaten for pace by Quinn. Instead it was Mark Cosgrove who led the assault, hitting Bopara out of the attack with successive fours in an over that cost 11.
Cosgrove hit Porter for 10 off two balls, including a six heaved over midwicket, to bring up the 50 partnership with Robson in 5.3 overs. Suddenly the target was below 50 in a dozen overs.
Robson reached his second half-century of the game when he flicked Quinn to square leg for a single. It had taken him 62 balls.
Cosgrove finally perished going for another big hit, caught low down at fine leg by Mickleburgh off Masters. His 38 had spanned just 21 balls and included six four and that six.
Former Essex captain Mark Pettini took up the cudgels, clubbing four boundaries in a 13-ball 16 before lobbing Bopara to ten Doeschate at mid-on. Two balls later ten Doeschate took his third catch, again at mid-on, removing Robson off Masters.
When Aadil Ali was caught behind off his glove to give Bopara a third wicket, Leicestershire were still 16 runs short with five overs remaining. But Neil Dexter and Eckersley carried them over the line without further alarm.