On a weather-shortened third day, Lancashire were bowled out for 407, with Sussex taking five wicket for 68 before making a faultless start to their second innings.
It means Lancashire’s bowlers will need to take ten wickets relatively cheaply on the final day today (on Sunday) if they are to guide their side to victory.
Only 42.3 overs were possible on Saturday and in the first half of that allotment the home side were dismissed for 407, giving them a first-innings lead of 106. Skipper Dane Vilas top-scored with 189.
However, openers Aaron Thomason and Tom Haines then resisted the Lancashire attack for 21 overs and Sussex were 38 without loss when bad light, sleet and rain ended play early on a bleak evening. That leaves the visitors with a deficit of 68 runs going into the final sessions of the game
In the first 90 minutes of play Lancashire added 68 runs for the loss of their last five wickets with skipper Vilas being last out for 189, caught at deep point by Aaron Thomason off Jack Carson to give the Ulsterman his second Championship wicket.
However, the day will be even more memorable for Sean Hunt, who trapped both Luke Wood and Danny Lamb leg before wicket and thus finished with three for 47 from 21 overs on his first-class debut.
A brace of mix-ups resulted in the run outs of Tom Bailey and Tom Hartley but by then Lancashire had collected a full haul of five batting points. England prospect Ollie Robinson finished with two for 69 from 23 overs and Carson two for 106 from 24.4.
Needing to bat for at least a day in order to avoid defeat, Sussex suffered no setbacks in reducing the deficit to 84 before tea and the openers maintained that resolute approach in the short session that the weather allowed on the resumption.
Sussex joint head coach Ian Salisbury said: "I was really proud of the boys today. We were put under the pump on Friday but they were magnificent today. We got things slightly wrong on Friday but they were looking to get over 150 ahead today and so to keep them down to what we did made me really happy.
"Although you are under the pump when you are a hundred behind we were still talking about what we needed to do to win the game. This evening we had a chance to get ahead of Lancashire until the weather intervened.
"We’ve had our eye on Sean Hunt for a while and I look at the learnings he and Jack Carson will have taken from yesterday. That and what they did today bodes well not just for this game but also for the future."