Tom Haines made 91 and there were half-centuries for skipper Tom Alsop, Fynn Hudson-Prentice and Jack Carson as Sussex recovered from a mini collapse.
They head into the final day against Worcestershire at Hove with a lead of 297 and four second-innings wickets in hand.
Haines and Tom Clark knocked off their first innings deficit of 62 in a confident opening stand of 143 that appeared to be putting their side in a strong position.
But 45 minutes after lunch suddenly there was some assistance in the 1st Central County Ground pitch for Worcestershire’s seamers.
Haines and Clark departed to successive balls and both James Coles and Oli Carter fell cheaply as four wickets went down in 27 deliveries for just four runs.
When Dan Ibrahim was fifth out for a skittish but in the circumstances vital run-a-ball 28, Sussex were only in front by 116.
But the obdurate Alsop and Hudson-Prentice tilted the balance back in their favour during a stand of 82 in 23 overs either side of tea.
Hudson-Prentice fell to Pakistan leg-spinner Usman Mir for 54 but Alsop was joined by Carson in another productive alliance, so far worth 99, with Sussex closing on 359 for six.
At the start of the day Worcestershire opener Jake Libby had been dismissed off the fourth ball without adding to his overnight 198.
Libby turned down a single to protect last man Ben Gibbon only to edge the next delivery low to Clark at second slip and give Ari Karvelas, who finished with 4 for 54, his fourth wicket with the second new ball. Worcestershire had lost their last seven for 44 in 12.2 overs.
The momentum seemed to be back with Sussex, especially when Clark and Haines rattled along at more than five an over before lunch.
Haines needed just 48 balls to complete his fifty, swatting away anything loose in his strong scoring areas on the leg side for the majority of his 10 fours.
Clark was more circumspect and nearly ran out his partner when going through for the single to get to his half-century.
Sussex were making serene progress but after lunch Worcestershire’s seamers tightened their lines and the tide briefly turned.
Haines will be disappointed with his dismissal to an indeterminate poke outside off stump to a channel ball which he guided to second slip, having hit 18 boundaries.
Matthew Waite picked up Clark with the next delivery off a very feint edge to wicketkeeper Gareth Roderick before Adam Finch dropped short and Brett D’Oliveira made good ground from mid-on to take a fine catch off Coles’s top-edged pull.
In his next over Finch pinned Carter with late inswing and although Ibrahim was dropped at slip by Adam Hose on 19, it didn’t prove expensive. In his next over Matthew Waite had him leg before and Worcestershire were on top.
But that was to be their last success for a while as Alsop and Hudson-Prentice rebuilt. Alsop was happy to drop anchor while Hudson-Prentice attacked at the other end.
He hit nine fours in his half-century and it was a surprise when he tried to cut a ball from Mir that he could have left alone and under-edged to the keeper.
Worcestershire took the second new ball when it became available but by then Jack Carson had joined his captain in another crucial partnership.
It had swelled to 99 by stumps with Carson reaching his fifty with a six off Mir – and with Alsop, who has batted four hours for his 67, playing an important role to set up what could be an enthralling final day when all results are possible.
