Essex 314 for 7 (Westley 110, Ryder 71, Zaidi 41, Foster 36*) beat Hampshire 310 for 4 (Wheater 90, Alsop 83 ret hurt, Dawson 70*) by three wickets
James Foster used his incredible experience to edge Essex over the line to get the Eagles off to the perfect Royal London One-Day Cup start - beating Hampshire by three wickets at the Ageas Bowl.
Veteran Foster scored a cool unbeaten 36 to help Essex score 29 from the last three overs - after Tom Westley had scored his third List A century.
Given 311 to chase after electing to bowl first, Essex attacked their reply, needing less than nine overs to reach fifty - but soon after lost Browne when he gave Ryan Stevenson his first List A wicket, caught by Adams at mid-wicket.
Westley and Jessie Ryder kept the score ticking over at exactly a run a ball - the former collecting a watchful half century from 66 balls, reaching the landmark with back-to-back leg-side clips. Ryder reached his fifty in a quicker 53 balls, before Westley was dropped on 69 by Liam Dawson at mid-on.
The duo were otherwise untroubled before Ryder was stumped by Wheater after a nice tempter by Mason Crane - the partnership ending on 143, the New Zealander scoring 71.
Bopara, run out by Crane, Lawrence and ten Doeschate, both caught behind, all fell within six runs to stunt Essex's progress and swing the game back towards the hosts.
Westley reach three figures with a brilliant on-drive down the ground, his third format ton, before he skied Andrew straight up, to return to the dressing room just one short of his best score.
Ashar Zaidi added another twist to the game, who along with Foster, swung hard in a 18 ball 30 stand but the former Sussex man's wicket, for 41.
But Foster and Masters ran and hit hard, along with some wayward death bowling to win with two balls to spare with a boundary over midwicket.
Earlier, Wheater's peerless 90 against his old county helped Hampshire to 310 for 4 - a seemingly winning score, the county never having failed to defend 300.
The wicket keeper put on a club record second wicket stand of 176 with Tom Alsop, who was forced to retire ill for 83.
Jimmy Adams had lasted until just the second over before he feathered an attempted pull shot through to Nick Browne at first slip.
Alsop and Wheater's stand proved relatively chanceless bar a couple of uppish strikes by both batsmen but safe from red shirted hands - and a six from Alsop which Dan Lawrence parried over the rope.
Both reached classy half centuries, Wheater from 71 balls before Alsop met him at the milestone little more five minutes later but in a speedier 57 deliveries.
Alsop in particular looked effortless at the crease, with the ball gliding off his bat beautifully on the back of his maiden first class fifty against Nottinghamshire a fortnight ago.
Wheater, a former Essex academy player, was faultless getting to 90 - summed up with a confident thrash through the leg side to the boundary - but he departed to end the stand 176 run stand miss-timing to Ravi Bopara at mid-off.
Skipper Sean Ervine and Liam Dawson then upped the run rate, with a 74 run partnership - the former eventually bowled by Ryan ten Doeschate.
Dawson did move to fifty, although lost Gareth Andrew to a boundary catch in the process, from 45 balls. Dawson ending his unbeaten 70 with a flurry of a delightful four and clubbed six, but it was not to be enough.