Centurion Francois du Plessis took just 19 balls to reach his second 50
Lancashire made up for two defeats by Essex in the group games by beating the Friends Provident Trophy holders by 67 runs to reach the semi-finals.
Francois du Plessis was the key to victory at Old Trafford, hitting 113 not out from 75 balls with seven sixes in Lancashire's total of 262-6.
Essex then rarely looked like reaching their target, despite 44 from Grant Flower and Ryan ten Doeschate's 41.
Spinner Gary Keedy took 4-45 as the visitors were bowled out for 195.
Lancashire now face a home semi-final date with Hampshire, conquerors of Middlesex, at Old Trafford, on 5 July.
In the other semi thrown up by Saturday evening's draw, Sussex, who beat Somerset at Taunton by six wickets, have earned home advantage against the winners of the delayed quarter-final between Gloucestershire and Nottinghamshire.
But Lancashire will be hard to stop if Du Plessis finds this sort of form again.
In hitting his second hundred in three innings, the South African reached three figures off only 71 balls to become the fastest one-day centurion of the English summer so far.
And it was his own record he broke, following the 76-ball ton he hammered against Derbyshire, his first for Lancashire, just nine days previously.
The 24-year-old went from 50 to 100 in only 19 balls to help accrue a match-turning 121 runs off the last 12 overs, including 63 runs off the five overs of batting powerplay.
Earlier, Mark Chilton (43) and Ashwell Prince (40) had been Lancashire's mainstays until du Plessis took over.
But, although four of the day's five top scorers were of South African descent, it was only the remarkable du Plessis who scored with any fluency on this Old Trafford wicket.
Having won the toss and put Lancashire in, Essex skipper Mark Pettini's decision looked justified when du Plessis came to the crease with Lancashire on 86-3 in the 27th over.
Opening bowler David Masters had put his side in healthy shape with a superb 10-over spell of 2-14.
He claimed the wickets of Paul Horton and Steven Croft, while part-time left-arm spinner Grant Flower also took 2-9 in three overs. And, at 154-5 in the 41st over, a third successive defeat to Essex in this competition for Group D winners Lancashire looked on the cards.
But, having reached his fifty in the 45th over off 52 balls, du Plessis finally set about the Essex bowling to stunningly transform the contest.
Lancashire then maintained their momentum with ball in hand, reducing the Eagles to 23-2 inside 10 overs.
England Test opener Alastair Cook (12) was the victim of a fine running catch by Sajid Mahmood at mid-on off the bowling of skipper Glen Chapple. And that man du Plessis was then crucially back in the action again.
This time it was with a precision throw to get rid of the in-form Varun Chopra, scorer of a half century in each of those two group wins, for a six-ball duck.
Mark Pettini (24), Matt Walker (19) and James Foster (10) all got starts. But, once Foster had gone to a diving catch by Chapple, Essex were 99-5 in the 29th over.
And when Flower was caught at midwicket, at 130-6, that was their final realistic hope gone, a few late blows from ten Doeschate merely delaying the inevitable.
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