Ian Ward made the second-highest score by a Sussex batsman in the 35-year history of the one-day game but it was not enough to avoid defeat in a run feast at Horsham.
The left-hander made a superb 136 off 120 balls, his second century in three days at Cricketfield Road, and looked to be leading Sussex to what would have been a sensational victory after second division leaders Leicestershire had piled up 324-4.
When Ward lost his off stump cutting slow left-armer Claude Henderson in the 39th over and Chris Adams was caught in the deep in the South African's next over the Foxes looked home and dry.
Sussex needed 15 runs off the last over but Michael Yardy, making his first appearance of the season, could only find the boundary off the final ball from Mark Cleary when six was needed and Leicestershire had squeezed home by one run in a breathless finish.
The pitch was as placid as the neighbouring strip on which more than 1,300 runs were scored in the Championship match, but nobody in a crowd close to 3,000 was complaining that it was too heavily weighted in favour of the batsmen as 52 boundaries and 15 sixes were scored.
Bowlers on both sides took some fearful tap, but Sussex also conceded 16 wides and that was to prove crucial.
Leicestershire's Brad Hodge would certainly have felt hard done by if his brilliant 154 not out had been made in a losing cause.
The Victorian did much as he pleased in reaching his second league hundred for the Foxes off 105 balls, but he then went into overdrive, adding a further 58 off his next 17 deliveries.
Nobody in the crowd was safe as Hodge flexed his powerful shoulders to smash 44 in boundaries in his third 50, including four more sixes, to eclipse the 140 made by Worcestershire's Glenn Turner on this ground in 1981 as the highest score against Sussex in the league.
It was also a new individual record for Leicestershire, beating Barry Duddleston's 152 against Lancashire in 1975.
Paul Nixon and Phil DeFreitas helped him plunder 116 off the last ten overs and for the Sussex attack there was no hiding place.
James Kirtley's eight overs cost 75 and the wicket of Nixon will have given him scant consolation. In two league matches since returning to the side Kirtley has conceded 145 runs from 17 overs, but he was not the only one to suffer.
Fourteen overs of spin from Mark Davis and an out-of-sorts Mushtaq Ahmed cost 108 and only Robin Martin-Jenkins escaped with his pride and figures intact. The all-rounder bowled a consistent line and was rewarded with the wicket of Darren Stevens in his fourth over.
It was another 26 overs before Sussex were celebrating again. Hodge and Darren Maddy added 165 for the second wicket but Maddy missed out on a deserved hundred, run out by a combination of Davis and Yardy.
There cannot have been many punters prepared to pile in on the bookmakers' teatime odds of 5-4 on a Sussex win, but for a long time Ward appeared to be pacing Sussex's reply to perfection.
Cleary deceived Matt Prior with a slower ball in the sixth over but Murray Goodwin proved the ideal foil for Ward as they added 143 in 22 overs for the second wicket.
Leicestershire skipper DeFreitas kept changing his bowlers, but it made no difference to Ward or Goodwin who kept up with a run rate of eight an over.
The momentum was maintained even after Jeremy Snape held a low catch off his own bowling to remove Goodwin for a run-a-ball 66.
Ward must have thought it was going to be his day when he was dropped in front of the pavilion by David Brignall on 110 in the 34th over and when the last ten overs began Sussex needed 103, Adams had joined Ward and was warming to the task.
It was down to 56 off six when Ward departed to a standing ovation, having hit ten fours and five sixes, and perhaps the key wicket was that of Adams as a relieved Brignall clung on to a skier at mid-wicket.
Yardy and Montgomerie did their best but making the highest total of any side batting second in league history will be little consolation today. Sussex have lost three out of four so far, but the annoying thing is that they could easily have a 100 per cent record.
There is nothing wrong with their one-day batting at the moment, but the same cannot be said of their bowling.
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