Sussex 368 (Wells 81, Zaidi 88, Yardy 56, Stone 5-48) and 209 for 3 (Wells 91*) lead Northamptonshire 294 (Keogh 129, Magoffin 5-51) by 283 runs
Sussex are within sight of finishing third place in Division One, having put themselves in a good position against bottom club Northamptonshire at Wantage Road.
The visitors only need to avoid defeat on Friday's final day and on 209 for 3 in their second innings with an overall lead of 283, they look well set to do at least that.
They had begun the day strongly, mopping up Northamptonshire's tail inside 80 minutes, for 294. Under cloudy skies, Neil Wagner was trapped in front for five - a decision he didn't appreciate - while Graeme White caught cheaply at slip, to give Australian seamer Steve Magoffin 72 wickets in another profitable season.
Wednesday's Northants centurion Rob Keogh added 23 for the tenth wicket with last man Azharullah, but when Keogh's 253-ball vigil for 129 was ended by a sharp Chris Nash catch off his own bowling, Northants were 74 behind. With the home side's attach offering little penetration, Luke Wells and Ed Joyce calmly added 83 for Sussex's second wicket after the early loss of Chris Nash.
A period of 10 overs after lunch produced just 12 runs, but Azharullah was recalled to bowl Joyce for 48, ending the Irishman's best first-class season in nearly a decade. After turning 36 on the opening day of this game, Joyce has scored 1454 first-class runs with eight centuries to his name in his best season since arriving from Middlesex in 2009.
But in his captain's absence, Wells powerfully hit the bad ball to reach 91 not out, including a six over mid-wicket off James Middlebrook, who is playing in his last game for the hosts. Wells, the 23-year-old son of former batsman Alan Wells has passed 50 on seven previous occasions in the championship, but he was prevented from passing three figures for the first time by bad light, which brought close half an hour early.
Add Sussex's highest score in the first innings and he has taken the chance against the bottom club to add lustre to his season's batting average.
Sussex do not need to force their hand to finish third, but if they do decide to declare with an overnight advantage of nearly 300, Magoffin is likely to be the spearhead. But, despite his highest wicket haul since joining Sussex in 2012, the veteran is not optimistic they can force a victory.
"It's a pretty dead track, to be honest. It's not easy for batsmen to bat on and it's certainly not easy for bowlers to feel as if they're ever likely to get a wicket,'' the Australian explained. ``You've got a bit of chance with the new ball but once 20 overs come off, it's a hard slog.''
Northants head coach David Ripley admitted it will be a relief to see the back of a turbulent 2014 which has seen them lose 12 times this season, eight of them by an innings.
"Last day of the season is always an emotional one with players leaving, so I just hope the players go out there and play some good cricket,'' he said. `"There's been lots of times when we've been down in the dumps having not played well. It'd be nice for the members and the players to sit down with a beer at the end of tomorrow, knowing that we've finished well.''