Essex 291 for 5 (ten Doeschate 119, Westley 111*) beat Yorkshire 290 for 5 (Leaning 111, Masters 4-34) by five wickets
Jack Leaning's first century for Yorkshire in any form of cricket could not prevent his side crashing to defeat against Essex in a high-scoring Royal London Cup match at Scarborough.
The 20-year-old's unbeaten 111 helped Yorkshire to a challenging 290 for 6 off their 50 overs - but Essex opener Tom Westley and captain Ryan ten Doeschate plundered centuries of their own to give their team a five-wicket success.
Westley and his captain shared a mammoth fifth-wicket stand of 209 in 31 overs - a partnership only eight runs short of being the highest against Yorkshire for any wicket in one-day cricket. Westley's 111 was also his maiden List A ton, while ten Doeschate blasted 119 from 101 balls with nine fours and six sixes.
Essex's victory, with 25 balls to spare, moved them joint top of the table with Yorkshire, leaving both sides in with an excellent chance of making it through to the quarter-finals.
Essex head coach Paul Grayson said: "At the halfway stage we were slightly disappointed that Yorkshire had reached 290 but it was a good pitch and we felt we just needed one or two batsmen to take some responsibility - and Westley and ten Doeschate did so. I think we are well placed to make the quarter-finals but one more win should guarantee it.''
Sunday's torrential rain left large areas of the pitch under four inches of water with the chances of play on the opening day of the 128th Festival looking slim - but groundsman John Dodds worked wonders to ensure a prompt start for the 3,200 crowd under sunny skies and in a strong wind.
Although Adam Lyth made promising progress early on for the hosts, Alex Lees departed for 8 in the sixth over, his off stump being knocked back by David Masters, the pick of the Essex attack with 4 for 34 runs from his 10 overs.
Kane Williamson lunged wildly at a ball outside off stump from Masters, only to edge to wicketkeeper James Foster, then when Lyth was run out for a run-a-ball 38 by a direct hit from Kishen Velani, Yorkshire were 55 for 3.
However, Leaning joined captain Andrew Gale and the fourth-wicket pair steadied things up with some careful batting - with Leaning's boundary over mid-on the first in 11 overs. Gale was in an hour before striking his first four but in the same over that he drove ten Doeschate over long on for six he slapped a catch to Tim Phillips at backward point and departed for 45.
Adil Rashid soon became Masters' third victim to make it 154 for 5 but Leaning and Andrew Hodd rapidly advanced the score with a quickfire partnership of 109 in 12 overs.
Leaning's 50 arrived off 67 balls and, warming to his theme, he blasted Phillips over midwicket for six and four before hoisting a Graham Napier full toss high over the rope.
Hodd got a six with a remarkable reverse slog-sweep at Reece Topley but in attempting another similar shot he was bowled by Masters for 42.
Essex had lost much of their earlier control and Napier sent down several wides, his final over of the innings costing 23 runs and providing consecutive sixes for Leaning, the first of which raised his century off 97 deliveries with four fours and three sixes.
The final ball was belted over square leg for six by Bresnan, 124 runs gushing from the last 10 overs and Napier's 10 overs costing him 84 wicketless runs. Leanings innings spanned 99 balls and he hit four fours and four sixes.
The visitors got off to a dreadful start, their first three wickets going down for 15 runs in six overs. Mark Pettini was caught behind off the third legitimate ball of the innings from Jack Brooks, before Greg Smith and Velani departed in consecutive overs from Bresnan.
Westley remained firm at the top of the order, however, while Jesse Ryder took the attack to Yorkshire in style, hitting six fours and a six in his 36 off 27 balls before slapping Richard Pyrah to Lyth at point.
Then ten Doeschate launched himself into the bowling by driving Leaning, Pyrah and Rashid for sixes - just beating Westley to the 50 mark despite batting for 14 fewer overs.
The stand reached 100 in 16 overs and Essex were left needing 123 to win off 20 overs, the target drawing closer as Westley struck Steven Patterson for six and ten Doeschate pulled Leaning over midwicket for another maximum.
The Powerplay was taken at 203 for 4 after 34 overs and Yorkshire were powerless to stop the onslaught, ten Doeschate dashing to his century from 86 balls with seven fours and six sixes and Westley following him to three figures from 120 deliveries with eight fours and a six.
The skipper eventually fell by mishooking Patterson to Lyth with the score on 277 but it was all too late for the home side and Westley finished things off with a six off Adil Rashid, who went for 71 runs from 8.5 overs.