Essex 334 for 6 (Westley 100, Bopara 92*, Cook 65) beat Somerset 262 (Allenby 77, Myburgh 57, Wagner 3-55, Harmer 3-56) by 72 runs
Tom Westley hit a century as Essex moved to within touching distance of a home semi-final with a 72-run Royal London One-Day Cup win over previous South Group leaders Somerset at Taunton.
A crowd of around 4,000 saw the visitors run up 334 for 6 from their 50 overs, Westley making exactly 100, Ravi Bopara 92 not out and Alastair Cook 65. Craig Overton and Roelof van der Merwe claimed two wickets each.
Somerset were given a lightning start in reply by Johann Myburgh (57). But despite a solid 77 from skipper Jim Allenby, they lost wickets regularly as Neil Wagner took 3 for 55 and Simon Harmer 3 for 56.
The result gave Essex a two-point lead over Somerset going into the final round of group games, so avoiding defeat in their final match against Kent at Canterbury would put them in the last four. Somerset may have to be content with a play-off spot.
After the early loss of Varun Chopra, Westley and Cook laid the foundation for a competitive Essex total with a second-wicket stand of 135 in 20 overs.
Cook reached an untroubled 54-ball half-century, featuring six boundaries, shortly after his partner had reached the same landmark off 42 deliveries, with 9 fours.
It was a surprise when the former England captain, who had looked in total command, departed with the total on 156, caught behind flashing at a short ball from Jamie Overton.
Westley was in full flow by then, moving fluently to three figures off 95 balls. Without addition he was bowled by left-arm spinner van der Merwe going down on one knee to swing to leg.
There was a second wicket for van der Merwe when Ryan ten Doeschate was bowled by a delivery that skidded through. At 215 for 4, Essex were in danger of falling short of 300.
That changed when Bopara and Ashar Zaidi added 88 in less than ten overs before the latter was well caught by Adam Hose at deep mid-wicket for a brisk 32.
Bopara went to his fifty with a straight six off Tim Groenewald, having faced 56 balls, and completed a valuable contribution with another maximum off the final ball of the innings from Craig Overton following a short rain break.
Somerset's reply began in sensational style as Myburgh, who survived the simplest of chances to Paul Walter at mid-on off the fifth ball of the innings, bowled by Matt Quinn, went on the charge.
The South African was on 48 by time the total reached 50 after just 4.4 overs and completed a thrilling 28-ball half-century, with 6 fours and 4 sixes, before holing out to Chopra on the mid-wicket boundary off Quinn.
Peter Trego, Steve Davies and James Hildreth fell cheaply as Essex began to peg the run-rate back. At the halfway stage of their innings Somerset were 158 for 4, with Allenby well set.
The captain took few risks moving to a 56-ball half-century and was looking on course for his first Somerset hundred at Taunton when he drove loosely at Neil Wagner and directed a simple catch to cover.
Adam Hose, looking to build on his maiden century in the previous game against Gloucestershire, looked good again before being bowled for 28 by an off-break from Simon Harmer, who sent back Jamie Overton in the same over, attempting a second six over mid-wicket.
With the required run-rate rising fast, van der Merwe and Groenewald perished to attacking blows. Some lusty hits from Craig Overton kept Somerset hopes alive before he was run out to finish the game.