Northamptonshire 97 and 137 for 4 (Gay 67) need a further 147 to beat Sussex 143 (White 4-23, Procter 3-37) and 237 (Simpson 40, Procter 4-45)
Sussex's bowlers struck crucial blows late in the day at Wantage Road, moving closer to securing a win and extending their lead in the Vitality County Championship Division Two.
Chasing a target of 284, Northamptonshire's Emilio Gay seemed to give his team a fighting chance with his 67 runs. However, both he and captain Luke Procter were dismissed within four balls, shifting the advantage firmly in favour of Sussex in this low-scoring game.
Nathan McAndrew, who trapped Gay's leg before, finished with figures of 2 for 37 as Northamptonshire ended the day on 137 for 4, still needing another 147 runs to achieve their first win of the season.
Earlier, Procter had a standout performance with the ball, taking 4 for 45, his best figures of the season, as Sussex were bowled out for 237 in their second innings. Ben Sanderson also contributed with 3 for 44.
The first day saw 22 wickets fall, and the trend seemed set to continue when Tom Alsop was caught behind off Sanderson in the first over, leaving Sussex's overnight lead unchanged at 129. Sanderson soon claimed nightwatchman Sean Hunt, and Raphy Weatherall along with Justin Broad exploited the surface's movement and bounce.
James Coles hit a series of elegant cover drives to the boundary, while Oli Carter, after a slow start, lofted Procter for a six over square leg. However, Procter responded by trapping Coles lbw.
Broad struck twice after lunch, removing Carter with a ball that seemed to be bounced, then dismissing Fynn Hudson-Prentice, who misjudged a cut shot and was caught at mid-off.
Northamptonshire missed several chances in Sussex's first innings, including a dropped catch by Gay off Sanderson and a missed run-out opportunity by Broad. McAndrew capitalised on these errors, hitting five boundaries in his valuable 22 before being out in a peculiar manner.
Procter dismissed the last two Sussex wickets quickly, including Simpson for 40, leaving Northamptonshire with nearly a session and a half to bat. They lost Ricardo Vasconcelos before tea, trapped by McAndrew's second delivery. However, Prithvi Shaw responded with stylish shots, partnering with Gay for a 69-run stand, the highest of the match.
Left-armer Hunt bowled Shaw for 37 and nearly dismissed Gay in the next over, but Simpson couldn't hold the catch. Gay reached his sixth half-century of the season but was soon dismissed along with Procter in successive overs. Offspinner Jack Carson induced Procter to edge to Simpson, and Gay's departure left Matthew Breetzke and Rob Keogh with the task of keeping Northamptonshire's hopes alive for another day.