Previous leaders Chichester had the worst of a draw against reigning champions Brighton & Hove, who remain bottom of the table, but just seven points separate the top six clubs as East Grinstead, Hastings, Preston Nomads and Three Bridges all enjoyed wins.
Sussex opener Chris Nash returned from the county's win at Nottingham to open the batting for his club and he hit 101, sharing an opening stand of 173 with fellow professional Michael Thornely, who made 114 not out in a Horsham total of 287-4 declared.
Hastings chased that lofty target in positive fashion and were rewarded with a seven-wicket as skipper Jason Finch hit 78 not out and both James Beany and Adam Makaraj-Newman each made 70.
A total of three runs more came at Fulking where Preston Nomads beat Cuckfield by 54 runs after rattling up 316-5 from 49.5 overs. Michael Gould struck 129 from just 139 balls and Jeremy Green made 94, the pair sharing a second wicket stand of 209.
Kashif Ibrahim with 34 and Joe Gatting 31 also made useful contributions as the runs flowed at more than six an over.
In reply Cuckfield were dismissed for 262 as Tasmanian Jason Shelton made 88, including six sixes, and Camran Mirza hit 85 not out. For Nomads, Ibrahim took 4-103 and there were two wickets each for Nick Hobbs and Ryad Emrit.
Sussex youngster Ollie Rayner had a fine game for Eastbourne but still ended a loser at East Grinstead. Eastbourne are still looking for their first win of the season. Rayner took 5-78 as East Grinstead made 233-8 thanks chiefly to Matt Kleinveldt (82) and Alex Edwards (78).
The county all-rounder then played beautifully to make 125 but his team-mates let him down as they tumbled to 190 all out at the hands of Sussex Academy player Will Adkin (5-55).
Three Bridges enjoyed their second win of the season as they bowled out hosts Stirlands for 173 in the 43rd over before sealing a six-wicket victory in eight overs more.
Sean Heather made 50 and Danny Cox 41 for Bridges as Paul Smith took 3-39 and there were three run outs. James Chadburn (78) and skipper Mark Newell (59no) then led the reply.
Champions Brighton are still looking for their first win of the season after being held to a draw at Chichester. Batting first Brighton made 264-6 in 55 overs, with skipper Patrick Spencer making 73 not out, Jordan Rollings 34 not out and youngsters Luke Wells and Matt Machan each 34.
The Chichester reply stuttered to 188-7 with 38 each from skipper Jez Goode and Mark Bamford, and 32 from Aussie Tim Bishop as Spencer took 3-44.
On Sunday the Sussex Development XI made 255-5 at East Grinstead as Stefan Piolet struck 126. The hosts were then 95-5 with former Sussex skipper Alan Wells unbeaten on 47 when torrential rain caused an abandonment in the 37th over.
Relegated Lewes Priory continue to lead the way in Division 2, their 44-run win at Steyning being their third in four starts, while Roffey opened their winning account and there were also wins for St James's and Bexhill.
Batting first at Steyning, Lewes were dismissed for 225 with Ed McGregor making 83 and Simon Allan 59. For the home side Brian Schultz took 4-55 and Ian Simpson 4-56.
Steyning were then dismissed for 181 as Michael Murray took 5-15 and Simon Birch 5-40.
In an East Sussex local derby, hosts Sidley made 249-9 as Elliot Bullock (87) and Pete King (52) led the way against a Bexhill attack in which Sean Austin took 3-49 and Simon Bulteil 3-63.
Bexhill then sealed a five-wicket win thanks to Jonathan Haffenden (68) and Malcolm Johnston (59) as King proved an unlucky loser, followi ng up his half century with 3-59.
In another derby clash, St James's Montefiore edged out Haywards Heath in a white knuckle clash. Heath were put in and bowled out for 185 in 46.2 overs as Derek Horsham took 3-8.
Saints squeezed home by two wickets, with Ashley Wallace proving the matchwinner with 66 not out. Charlie Dowdall took 4-54.
Roffey produced the surprise of the day as they won by 39 runs at Littlehampton after posting a total of 229-8 with 52 from Chris Webb and 38 from Mark Pavlovic. Chris Hanks took 4-71 and Benn Challen 3-37.
Littlehampton were then bowled out for 190 despite 52 from Chris Kirkham and 35 from James Askew. For Roffey Luke Barnard (4-25) and skipper Josh Fleming (3-16) did the damage.
Worthing skipper Dominic Clapp hit the highest score of the season with 200 not out in a total of 301-3 at home to Pulborough. He scored his runs from just 172 balls and was well supported by opening partner John Kaye, who made 44.
In reply Pulborough reached 249-2 but never made a serious challenge for victory as Rohit Jagota hit 125 not out and Elliot Cloy made 56. On the day, each wicket averaged an amazing 110.