Essex 324 (Ryder 87, Westley 74, Bopara 51) beat Hampshire 319 for 9 (Wheater 135, Vince 79, Masters 3-51) by five runs
Essex clinched their place in the Royal London Cup quarter-finals with a thrilling five-run win over Hampshire at Chelmsford.
Half-centuries from Jesse Ryder, Tom Westley and Ravi Bopara had laid the foundation for their imposing total - but at one stage it did not look enough as Adam Wheater showed his liking for the home attack.
He struck a brilliant 135 runs from only 97 deliveries, which followed the century he scored against his former colleagues when the sides met in the County Championship at Colchester last month.
When Wheater was out, caught at mid-off by Graham Napier off Ryan ten Doeschate, his side needed just 55 runs from 10 overs, with six wickets intact.
His main accomplice was James Vince, the pair sharing in a partnership of 198 in 28 overs, a county record for any partnership against Essex in List A matches. They would have expected that stand to have carried their side to victory.
Essex skipper Ryan ten Doeschate said: "It was a fantastic win. While Wheater and Vince were together, it did not look good for us but we kept on fighting to get over the line.
"The first part of our mission has been accomplished by getting into the quarter-finals but now we want victory at Northampton to put us in line for a home draw.''
Vince made 79 from 83 deliveries, 26 of his runs coming in boundaries, before he fell to a superb catch by Tim Phillips at long-on - providing David Masters with one of his three wickets.
But Essex made things difficult for themselves by spurning opportunities to remove both batsmen.
Wheater, on 73, was let off as James Foster missed a comparatively easy stumping off Phillips with the total on 175, while Vince was put down by Kishen Velani on the square-leg boundary in the next over when attacking ten Doeschate.
The pair had come together after Masters had removed openers Jimmy Adams and Michael Carberry in the space of three deliveries with the total on 48 in the ninth over.
After Wheater's dismissal, the Hampshire innings disintegrated and they fell behind the asking rate in the face of some fine ground fielding and tight bowling.
They lost five more wickets while adding 43 runs, including that of Chris Wood - who was run out by ten Doeschate off the penultimate ball.
James Tomlinson arrived with six needed to tie the match but he could only manage a single, ending any hopes Hampshire entertained by that stage.
Masters finished with 3 for 51 and Napier 2 for 58, the Essex attack having been handicapped earlier by Tymal Mills leaving the field with a muscle injury during his third over.
Essex had been put into bat and Westley and Pettini gave them a solid start with a stand of 80 before the latter was caught down the leg side by Wheater off Matt Coles with the last delivery of the 15th over.
Westley, fresh from his unbeaten century on Monday against Yorkshire, then took part in another productive stand of 81 in 13 overs with Ryder as they underlined the friendly nature of the pitch.
Both were run out. Westley had made 74 from 79 balls with the help of 10 fours when he failed to beat the arm of Danny Briggs attempting a second run, while a Wood throw from backward point saw off Ryder as he called Bopara for a single with the total on 219.
Ryder's approach brought him four sixes amongst his 13 boundaries, while contributing 87 from just 61 deliveries. After that, wickets fell steadily as Essex tried to increase their tempo further. Among them was Bopara, whose 51 from 52 deliveries contained just three fours, until Wood took a fine catch at mid-on to provide Tomlinson with his solitary success.
The last 10 overs of the innings produced only 56 runs for the home side at a cost of five wickets, all of which fell in the final eight overs.
Wheater said: "It was disappointing to lose after coming so close and being well placed to win. But now we can focus on our four-day game in the hope of getting promotion.''