Duckett had signed off 2015 with a century at The Kia Oval and picked up where he left off with a 139-ball century.
His 178 not out was a first-class best and allowed Northamptonshire to enjoy a fine opening to the new season against one of the favourites for the Specsavers County Championship Division Two title.
Duckett was his dominant best in the morning session, using any inch of width to slash behind square on the off side.
There were moments of fortune – two wild drives flew over the slips before he raised a half-century in 60 balls, while an under-edged cut brought him within touching distance of a century, which he reached with an ease through extra-cover.
He also swept the spinners with precision. Two of them operated in tandem for periods – a most unexpected sight for an April Championship match but such was the unresponsive nature of the pitch that stand-in Sussex captain Ben Brown had little choice but to use Danny Briggs and Luke Wells for 31 of the day’s 82 overs.
— NCCC (@NorthantsCCC) April 10, 2016
Duckett responded by lifting Briggs over long-on for his only six, while Wells sent down too many loose deliveries to be a threat.
The only period where Sussex sustained any pressure was shortly after lunch when 15 runs came in 10 overs and Steve Magoffin slid a delivery into Jake Libby to win an lbw decision.
Libby had joined on-loan from Nottinghamshire on Friday as injury cover and showed a competent technique for his 42 – but he should have been taken by Ross Taylor at first-slip when on just 12.
Duckett and Libby shared an opening stand of 130 but Northants captain Alex Wakely arrived to build another big stand. He and Duckett weathered the brief period of Sussex pressure to add 123 for the second wicket.
Wakely opened his campaign with a smart half-century and some pleasant straight driving but would have cursed his dismissal – playing round a Briggs delivery to be lbw for 51.
— Ross Taylor (@RossLTaylor) April 10, 2016
Duckett said: “I’m pretty chuffed. The job isn’t done, I’ve still got to try and go past 200 tomorrow. There were games last year when I was frustrated, getting out when set to go and make a huge score so it’s a great chance to do it on day two.
“There was a bit of nip early on and Steve Magoffin bowled really well. But the plan was to play as straight as possible and stay patient and we managed to do that.”
Mark Davis, the Sussex head coach, said: “It was a tough day but we stuck to our task very well. Our fielding was outstanding, we fielded with energy and skill.
“It wasn’t an obvious bowl first – we thought we’d look at the wicket, we saw the pitch was slightly damp and thought the best time to take wickets was first thing. It’s a good pitch and they played really well.”