Sussex piled more misery on Northamptonshire as they pushed bowled them out for just 116 after lunch at Hove.
Steve Magoffin added to the three wickets he took in the morning session by taking another two to leave the visitors trailing by 289 on first innings.
Matt Hobden also struck twice while there were wickets for Lewis Hatchett, Steff Piolet and Ashar Zaidi with Sussex almost certain to enforce the follow-on.
Magoffin had reduced Northants to 1-3 at lunch and they lost another quick wicket straight after the restart as Hobden trapped Rob Newton with his first legitimate ball.
That became 20-5 when Magoffin had Kyle Coetzer brilliantly caught behind for nine and 34-6 when Hatchett switched to the Sea End to remove Ben Duckett for seven.
James Middlebrook top scored with 28 before he edged Magoffin’s third ball back into the attack to Joyce at first slip. It was Magoffin’s third five-wicket haul of the season and the 16th of his career.
Piolet picked up the wicket of Andrew Hall with his second ball and Hobden trapped David Willey on the crease with a ball which kept low.
Steven Crook counter-attacked by hitting James Tredwell for six but holed out at long-on to Ashar Zaidi for 24 just before tea.