Anuj Dal scored an unbeaten 146 as Derbyshire declared on 551-8 just before tea to give the visitors the advantage on day 2.
In response, Tom Alsop and Mohammad Rizwan battled away in the final session, putting on a third wicket stand of 95, with Alsop eventually being dismissed for 44.
Sussex started the morning session well again, dismissing centurion Wayne Madsen for 176 after he was caught in the slips by Tom Alsop off the bowling of Jack Brooks. Hayden Kerr was then trapped lbw by Delray Rawlins, but Anuj Dal fought back to put on the highest first-class score of his career.
Rawlins followed up with another lbw, dismissing all-round Alex Thomson for 27 but Mark Watt managed to frustrate the Sussex bowling attack as he and Dal put on a partnership of 108 before the visitors declared on 551-8.
Derbyshire declared just before tea, forcing Sussex into batting for around 30 minutes before the final interval and the decision paid dividends.
Sussex lost Ali Orr and Tom Haines before Tom Alsop and Mohammad Rizwan came to the crease and stabilised the home side’s innings.
Rizwan has been a shining light in the Sussex team since joining, having played some outstanding innings in both T20 and County Championship cricket. The overseas wicket-keeper batter made another half-century in difficult circumstances and closed the day on 54 not-out.
Alsop made a tough-fought 44 from 86 deliveries but was caught by Leus du Plooy off the bowling of Luis Reece with 7 overs remaining in the day.
Rizwan and fellow wicket-keeper-batter Oli Carter will return to the crease tomorrow morning after we closed on 142-3 at stumps.