Saif Zaib, Adam Rossington hand Northamptonshire healthy first-innings lead
Sussex 106 and 154 for 4 (van Zyl 71*) trail Northamptonshire 441 for 9 declared (Zaib 135, Keogh 87, Robinson 5-58) by 181 runs
Saif Zaib posted a wonderful maiden first-class century before Ben Sanderson and Gareth Berg once again knocked over the Sussex top order in the LV= Insurance County Championship.
Talented left-hander Zaib, who has been at Northamptonshire since he was 15, collected a commanding 135 as he and Adam Rossington, 87, put on 198 together to help the hosts to a massive 335-run first-innings lead.
Sanderson and Berg, who shared all 10 of the first-innings wickets equally, then claimed two scalps apiece to leave Sussex 68 for 4.
But Stiaan van Zyl's stylish 71 not out helped Sussex recover to 154 for 4 by the close at Wantage Road, still 181 runs behind Northants.
Northants added 227 runs in 51 overs as Zaib, Rossington, Berg and Tom Taylor all found scoring simple against a largely toothless attack.
The caveat to that was Ollie Robinson. The quick wouldn't have done his England Test hopes any harm in front of ECB Performance Director Mo Bobat as he looked a constant threat during his 5 for 58 - the 16th five-for of his career.
Zaib and Rossington were the morning protagonists as their partnership grew to 198, one short of the county sixth-wicket record against Sussex.
Milestones came in rapid succession as the skipper reached his third half-century in five innings this season in 90 balls, before Zaib converted to a hundred in 176 balls.
They fell either side of the second new ball as Zaib was lbw sweeping and Rossington miscued a pull to mid-on, with Wayne Parnell falling quickly after to give Robinson his fifth.
But Taylor and Berg crashed Northants over the 400-run batting point mark in a 63-run blast, before the former smashed 21 in two overs, after Berg had fallen, to bring the declaration on 416 for 9.
Faced with a long task to reach parity, Sussex began their second innings with intent but soon collapsed to 68 for 4 in the face of Berg and Sanderson.
First-innings golden duck casualty Tom Haines raced to 18 off 17 before he expansively slashed Berg to a juggling Taylor at second slip.
Aaron Thomason edged Sanderson thickly to fourth slip, Australian debutant Travis Head under edged a Berg short ball behind and Tom Clark saw his off-stump bail flicked off by Sanderson.
South African van Zyl was dropped guiding to fourth slip on 19 but was otherwise at ease with Ben Brown, with a penchant for cover and straight drives. It was therefore little surprise that his 91-ball half-century was brought up with a beautifully timed straight drive, as he and Brown's partnership blossomed to 86 by the end of play.