Delray Rawlins and his Sussex teammates were denied consecutive victories in the Royal London One-Day Cup by Hampshire all-rounder Gareth Bergís match-winning performance that saw the visitors sneak a thrilling encounter by two-wickets with four balls to spare at The 1st Central County Ground, Hove.
Sussex looked favorites when Hashim Amla was sixth out for 63 to leave Hampshire needing 118 from 15.5 overs to overhaul Sussexís 250.
But Lewis McManus and Berg thrashed 59 from 37 balls for the seventh wicket and although McManus was caught behind in the 41st over with 57 still needed Berg unleashed some powerful shots and farmed the strike effectively, although he could have been run out on 47 when he slipped backing up too far only for David Wiese to miss with his throw at the stumps.
There was another twist when Berg, who had earlier taken 3 for 51, was caught at deep mid-wicket in the 47th over off Ollie Robinson for 65, made from 37 deliveries with eight fours and two sixes, to leave his side needing 15 off 20 balls.
But Mason Crane and Reece Topley kept their nerve and when Mason drilled Robinson back down the ground for a precious boundary, it meant three were needed off the final over.
Topley scrambled a leg bye off the first ball from Ishant Sharma then Crane crashed the next to the cover rope to seal victory.
Amla had set things up with 63 from 83 balls. The South African could only hit three boundaries but he held the innings together and allowed McManus and Berg to take the initiative.
Rawlins bowled 4-0-26-0.
Earlier, Sussex batsman Harry Finch had made his maiden one-day hundred a fortnight after scoring his first Championship century on the same ground.
In a well-paced knock, Finch hit eight fours and two sixes as he provided the glue for the Sussex innings. He was eventually bowled by Berg for 108 off 142 balls.
Topley, playing his first one-day game for more than a year after a succession of injuries, picked up Wells in his first over but Finch and Luke Wright, who was playing his 200th List A game, added 105 in 22 overs before Wright (56) top edged a sweep at Crane.
David Wiese, with 29 off 20 balls, hit out towards the end of the innings to take Sussex to a total that always looked competitive on a pitch on which it became increasingly difficult to time the ball.
Robinson picked Rilee Rossouw (0) and James Vince (9) in an excellent new-ball spell and Sussex seemed to be in control before McManus and Berg turned the tables.
Robinsonís 3-57 means he has six wickets from his two matches in this yearís tournament so far.
Rawlins also went without scoring, he got an edge that was taken low down by 'keeper McManus.
Sussexís next match in the competition is against Somerset at The Cooper Associates County Ground, Taunton on Tuesday.