Gloucestershire 183 and 108 for 6 (van Zyl 3-16) require a further 59 runs to beat Sussex 145 and 204 (Finch 48, Worrall 4-45 )
Four wickets in 17 balls boosted Sussex's chances of claiming their first Championship win of the season against Gloucestershire at Hove.
Chasing 167 to win, Gloucestershire looked in control when they reached 69 for 1. But Stiaan van Zyl picked up two wickets in successive balls and then had Jack Taylor caught behind. Ollie Robinson claimed the key wicket of opener Benny Howell for 45 as Gloucestershire slumped to 72 for 5.
As the light deteriorated, Sussex were forced to bring on left-arm spinner Danny Briggs who removed Graeme van Burren after van Burren had added 23 with Ryan Higgins. But conditions got worse and when the players came off because of bad light Gloucestershire were 108 for 6, 59 runs from victory with Sussex requiring four more wickets.
A dire forecast for Monday suggests the game might end in a draw which would be a pity. Both sides had periods of control on an absorbing third day. Although the odd ball misbehaved the pitch played pretty well but overhead conditions were poor throughout with the lights on from the start.
Sussex resumed on 51 for 2 and lost wickets at regular intervals. Opener Harry Finch batted through the morning session and for a while he and van Zyl looked to be putting Sussex in a decent position. They then lost van Zyl, who nicked Dan Worrall to slip, and Luke Wright, lbw to Higgins, in successive overs and before lunch Ben Brown (16) and Michael Burgess (17) had also departed.
Finch played the anchor role to perfection, coming out his shell to pull Jack Taylor for six, before losing his middle stump to Worrall for 48 after lunch. The Australian ended the innings when he had Ishant Sharma lbw to finish with 4 for 45 while Higgins' three wickets gave him a match haul of 8 for 86.
Gloucestershire lost skipper Chris Dent (13) when Robinson beat him with late seam movement but Howell and Roderick appeared to be putting their side in a strong position with a stand of 45 in 13 overs.
Then van Zyl nipped one away and Roderick was caught behind to give Ben Brown his 300th first-class catch and Jack Bracey lost his off stump to the next ball. Howells' patient innings was ended by another good ball from Robinson before van Zyl picked up Taylor.
Gloucestershire regrouped through Higgins and van Burren but Briggs deceived van Burren with his arm ball to make Sussex favourites again shortly before conditions got too bad.