Matt Machan, an ex-Sussex and Scotland batsman who was part of the Priory side which won the league in 2011, made 106 off 97 balls to set visitors Brighton & Hove on the way to a 30-run win.
On a day when more than 560 runs were scored on a great batting wicket at Horntye Park, Brighton amassed 297-4 from their 50 overs after winning the toss and choosing to bat.
Priory spinner Josh Beeslee (9-1-58-1) bowled Isaac Taylor for 15 with the score 35, but Machan and Bryce Hounsome proceeded to forge a huge second wicket partnership of 144.
Adam Pye (10-0-66-1) eventually bowled Hounsome for 88 and James Pooley (4-0-23-1) pinned Simon Hetherton leg before 12 runs later.
Machan went on to complete his ton before being caught by Greg Devlin off John Morgan (10-0-47-1) to leave Brighton 233-4. Chris Grammer (31 not out off 28 balls) and Waheed Safi (41 not out off 30 balls) put on 64 unbroken runs for the fifth wicket as Brighton got close to 300.
It was always going to be a massive ask for Priory to chase down the runs, but they made a pretty decent fist of it, being bowled out for 267 in 48.2 overs.
Although Priory got off to a bad start, losing a wicket before a run had been scored, they fought back superbly with a second wicket stand of 107 between Joe Billings and captain Tom Gillespie.
Billings in the end was trapped leg before for 49 off just 41 balls (with nine fours) and Gillespie departed 13 runs later, also to Jonathan Goldstraw, for 65 off 53 balls (with 10 fours and a six). When Greg Devlin was bowled by Stan Harrison for five, Priory were 122-4.
Their hopes of victory might have been all but over, but Priory rallied strongly with a sixth wicket partnership of 69 between Morgan and Jake Woolley before Morgan was caught by Machan off Kieran Buck for 28.
Priory lost a couple more wickets fairly quickly to sit 204-7. Woolley brought up his fifty shortly before Pye was run out for 15 off nine balls with the score 229.
Beeslee and Woolley took the total past 250 before the former was stumped off Simon Hetherton, who later claimed the final wicket to finish with 3-39. Woolley remained unbeaten on 76 from 95 deliveries (with six fours and a six) - his highest first team score.
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