Sussex 178 and 70 for 0 trail Northamptonshire 478 for 5 dec (Rossington 138*, Keogh 75, Duckett 72, Murphy 60*, Terry 54) by 230 runs
A career-best 138 not out from Adam Rossington has put Northamptonshire in a position to push for their first win of the season in the Specsavers County Championship.
Rossington, who was playing as a specialist batsman because of an injury to his left hand, hit five sixes and 16 fours in an impressive display to help his side establish a first innings lead of 300 against Sussex at Arundel.
They declared on 478 for 5 but Sussex openers Ed Joyce and Chris Nash successfully negotiated 24 hours before stumps to reach 70 without loss, still 230 behind with a day to go.
The experienced pair adopted a risk-free approach against attacking fields with Nash unbeaten on 36 and Joyce 28 not out.
Rossington had earlier helped Northants speed towards their declaration 25 minutes after tea on the third day in an unbroken stand of 199 in 39 overs with wicketkeeper David Murphy, a sixth wicket county record against Sussex, beating the 178 by Rob Bailey and David Ripley at Northampton in 1990.
The former Middlesex batsman was particularly savage on legspinner Luke Wells, hitting him for four sixes before swiping Ajmal Shahzad over the long-on rope. Murphy gave him good support with an unbeaten 60 as batting conditions eased on a pitch lacking pace.
Sussex had a glimmer of hope just after lunch when they reduced Northants to 279 for 5, but on a wicket offering their seam attack little margin for error they failed to make the most of the second new ball as Rossington grew in confidence.
The day had started well for Sussex. Ollie Robinson drew Alex Wakely with his first ball of the second day and had the Northants' skipper caught behind, but the visitors regained the initiative in a punishing fourth-wicket alliance of 119 in 27.3 overs between Rob Keogh and debutant Sean Terry.
Former Hampshire batsman Terry, who is on non-contract terms, improved his chances of a permanent deal by scoring 54 only to give it away just before lunch when he hit Harry Finch to deep midwicket.
Keogh batted well for his 75, compiled in just under three hours with 11 fours, only to lose concentration just after lunch when he tamely prodded forward to Shahzad and was caught behind.
Northamptonshire's lead was 101 at that point but Rossington and Murphy extended it impressively during the afternoon session.