Steven Croft and Karl Brown put on a magnificent unbeaten century stand for the third wicket to lead Lancashire to an eight-wicket victory in their Clydesdale Bank40 Group A match at Chelmsford.
The visitors passed their opponents' 186 all out total with 21 deliveries to spare and in doing so booked a home tie in the semi-finals.
Croft made 66 and Brown 87 as they put on 160 in 30 overs to make sure the earlier efforts of Oliver Newby with the ball were rewarded. The medium-pacer emerged with figures of five for 35, his career-best in one-day cricket, before his team-mates saw Lancashire to 187 for two.
Essex had looked set for a much more formidable total as openers Tom Westley and Mark Pettini moved to 47 in the 10th over without looking in any trouble. But Newby removed both in his first two overs and then put the home side firmly on the back foot at 70 for three when he also got rid of Greg Smith.
He later returned to remove Adam Wheater and Maurice Chambers as Essex's batsmen failed to make the most of decent starts. Five of them were dismissed in the 20s, among them Owais Shah, who was the victim of a brilliant piece of work in the field by Brown, scoring a direct hit from deep extra cover which proved the highlight of fine fielding throughout.
Paul Horton pulled off three catches, one a superb effort to get rid of Adam Wheater, while the athleticism of his colleagues cut off many strokes that looked destined for the boundary.
When they responded, the visitors did not get off to the best of starts, losing openers Stephen Moore and Ashwell Prince cheaply.
But Croft and Brown were to regain the initiative with a dominant third-wicket partnership full of controlled aggression.
James Foster juggled his attack to no avail in his attempts to make a breakthrough. But the pair scored freely against both pace and spin, while putting together Lancashire's highest third-wicket stand in limited-overs against Essex, beating the 114 between Clive Lloyd and Neil Fairbrother in 1986.
Brown's effort was his season's highest in all forms of cricket and contained eight fours and three sixes and spanned 97 balls. Croft's innings lasted 98 deliveries and he struck five fours to leave Essex reflecting on their fifth defeat of the season in the competition, while their opponents completed their sixth win in as many games in the CB40.