Sussex 407 for 5 (Simpson 112*, Hughes 83, Clark 73*, Crocombe 54) lead Glamorgan 186 (Carlson 56, Unadkat 4-52) by 221 runs
Skipper John Simpson became the first Sussex batter for ten years to score five Vitality County Championship hundreds in a season as the second division leaders took charge against Glamorgan at Hove.
Simpson also passed 1,000 runs in his unbeaten 112 as Sussex responded to Glamorgan's under-par 186 with 407 for 5, a lead of 221.
With promotion rivals Yorkshire and Middlesex in position to win their games, it is crucial that Sussex do the same and apart from a careless hour before lunch, when they lost four wickets for 25, they were in control at the 1st Central County Ground.
Simpson has so far added 194 for the fifth wicket with Tom Clark, who played his part with an unbeaten 73, and just before bad light intervened at 4.20pm the pair took 19 off ten balls to ensure Sussex collected a fourth batting point.
Ed Joyce - who also left Middlesex to move to the south coast - was the last batter as prolific as Simpson has been in what is his first year with Sussex, and not for the first time this season he made sure his team didn't squander a good position.
At the start of the day Daniel Hughes and nightwatchman Henry Crocombe extended their second-wicket stand to 78 and their only alarm came when Dan Douthwaite put down a tough chance at midwicket when Crocombe was on 25.
Hughes looked on course for back-to-back hundreds but on 83 (14 fours) he chipped a straightforward catch to midwicket off Douthwaite, who was Glamorgan's best bowler on a tough day. Crocombe forced Douthwaite through the covers for his seventh boundary to bring up a maiden first-class fifty before he was taken at short leg attacking off-spinner Ben Kellaway.
The ambidextrous Kellaway, who later in the day switched briefly to bowling left-arm spin, had Tom Alsop caught at slip pushing forward and James Coles, the only right-hander in Sussex's top seven, wafted at a ball he could have ignored in the last over before lunch to give Douthwaite his second wicket.
Clark's fifty was only his third of a difficult season and here he was tied down early on by Kellaway in particular. He didn't score a boundary until his 58th ball when he cut Kellaway through backward point but with Simpson counter-attacking at the other end Clark grew in confidence and Sussex assumed control.
Glamorgan only bowled eight overs of seam with the new ball before Kellaway was back on again at the sea end and the 20-year-old from Newport, who is playing only his fourth first-class match, impressed with his control on a surface offering some turn.
But as the sixth-wicket stand prospered the only question was how many batting bonus points Sussex could secure in 110 overs. Simpson took two boundaries off Kellaway, the second of which took the left-hander to his 15th first-class hundred, as ten came off the 109th over and a pulled six off Ned Leonard from the first delivery of the next followed by three singles took them to a fourth point with two balls to spare.
By then the light was fading and shortly afterwards umpires Nigel Llong and Sue Redfern took the players off but with two days left and a good forecast Sussex will feel they have enough time to press home their advantage and secure a seventh win of the season.
