Chris Summers hit the first century in division one this season, but it may not be enough to save Southwick from the drop.
The 19-year-old right-hander made 101 not out from just 67 balls to steer his lowly side to a shock victory against title-chasing Pagham.
Summers smashed 14 fours and four sixes with his 3.5 pound bat, but Wickers remain second from bottom.
Nick King (66) and Steve Martin (46) also contributed useful runs in a 219-1 reply after Pagham made 215-8, which included 52 from Chris Hayden.
Southwick skipper Mark O'Keefe was delighted with victory but realises it will probably not be enough to retain their place in the top flight.
He said: "We hammered them, it was undoubtedly our best performance of the season.
"Chris was magnificent and his knock shows we have a batsman, at long last, who can produce the goods."
Defeat is a massive blow to Pagham's hopes of a first championship.
They beat Findon last month to go top but the champions are now 26 points clear at the summit with four games left.
Burgess Hill remain a place above Wickers after winning at doomed Eastergate.
The bottom club were all out for 87 as Dave Pratt took 4-28. In reply, Hill made 91-5.
Clymping continued their revival and are fourth ahead of Lindfield following a two-wicket win at home against Ifield.
Graham Dean (93) and Dale Rachow (55) helped the visitors to 202-7 but Karl Pierce smashed an unbeaten 90 as Clymping (205-8) won with three balls to spare.
Arundel's match at home to Findon was cancelled.
Broadwater edged closer to the division two title with a hard-fought victory over struggling Henfield.
Batting first, Broadwater were 35-3 off nine overs before a 75-run partnership between Joe Owen (55) and Ben McKenzie steadied the innings and helped the leaders to 117 -6.
Spinner Tim Parsons claimed 5-33 off 12 overs to leave Broadwater all out for 169.
Henfield began badly when Chris Crampton was out first ball after an inside edge and were all out for 136. Tony Alcott took 4-23.
The league's leading wicket taker, Joe Owen, finished empty-handed for the first time this season.
Ansty climb to third in the league with a ten-wicket victory in their rain-affected match at bottom club Petworth Park.
Home skipper Nigel Scutt won the toss and elected to batted. Scutt hit 60 but Park were all out for 142. Shaun Janman (4-46), Barry Crouch (3-30) and Chris Nutter (2-22) bowled well.
Ansty clinched victory in just 22.3 overs as Richard Nichols hit an unbeaten 83 and David Croydon scored a first XI personal best of 55 not out.
Tom Filby, 15, made his first team debut for Barns Green and took 5-38 in the draw at home to Crawley. Olly Lampkin (49) and Martin Hennock (40) helped Green to 185-7. In reply, Crawley closed four runs short on 182-9, Dave Gillett made 47.
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