Sussex 223 for 9 (Machan 68, Joyce 58, Gurney 5-48) beat Nottinghamshire 173 (Taylor 48, Patel 45, Liddle 3-21) by 50 runs
Sussex ended Nottinghamshire's unbeaten start to the Yorkshire Bank 40 campaign when they toppled the Group A leaders by 50 runs at Trent Bridge in front of a crowd of 8,000 attracted by discounted admission prices.
After being tasked with scoring 224 to maintain their 100 per cent record, Notts' much-vaunted top order failed to deliver for the first time this season.
James Taylor top-scored for the home side with 48 and Samit Patel made 45 as Notts were bowled out for only 173 with the last seven wickets falling for 47 in only 10 overs.
Nottinghamshire remain favourites to win the group but will need to lick their wounds and recuperate after their winning streak ended at seven consecutive victories.
Earlier, after electing to bat first, Sussex reached 223 for 9 with Matt Machan making 68, hopes of a bigger total stunted by the performance of Harry Gurney who took 5 for 48.
Gurney removed both Luke Wright and Rory Hamilton-Brown in his opening burst before returning to claim three more wickets, including Machan, who had shared in a stand of 90 with Ed Joyce for the fourth wicket.
Many in the highest YB40 crowd of the season had come to hail the return of David Hussey to the Nottinghamshire ranks.
The Australian, returning as the county's overseas player, took an early catch but then blotted his copybook by spilling Joyce on 20. The Sussex skipper went on to make 58.
Machan's own 50 had come from 55 balls faced and included three fours and two maximums. Left with the lower order after Joe Gatting and Ben Brown had fallen cheaply, he tried to bolster the total by attempting some unorthodox deflections and ramp shots before his luck finally ran out and he was bowled.
Matthew Hobden, a 20-year old seamer from Eastbourne, began his debut in style for Sussex by bowling Michael Lumb in the fourth over.
The out-of-form Alex Hales failed again, making only seven before driving Andrew Miller to point where Hamilton-Brown took the first of his four catches.
Patel skied Chris Liddle, who took three for 21, to mid-off and Hussey only made 14 before the innings collapsed in dramatic fashion with 4.1 overs unbowled.