Derbyshire 274 (Madsen 97, Burgoyne 62*, Jordan 3-73) and 174 for 1 (Slater 66*, Madsen 62*) beat Sussex 314 (Wells 110) and 133 (Wright 49, Groenewald 5-40) by nine wickets
Derbyshire claimed their first Championship win of the season when they eased to a nine-wicket win over title challengers Sussex with 11 overs to spare on the third day, their first win at Hove since 1996.
It was a potentially fatal blow to Sussex's hopes of winning their fourth title since 2003. They have not won a Championship match at Hove for a year and they were always up against it after they were bowled out for just 133 in their second innings, with Tim Groenewald taking 5 for 40.
That left Derbyshire needing 174 for victory and they got there in 42.3 overs thanks to an unbroken stand of 102 between skipper Wayne Madsen and opener Ben Slater, who both made unbeaten half-centuries.
Sussex had resumed on 11 for 1, a lead of 51, but the rot set in after they lost their second wicket to the third ball of the day when Luke Wells thick-edged Groenewald to Peter Burgoyne at third slip.
Michael Yardy was third out at 22 in the 11th over when he edged a sharply bouncing delivery from Mark Footitt to the wicketkeeper and, without addition, nightwatchman Lewis Hatchett shuffled across his stumps and was lbw to Groenewald. When Rory Hamilton-Brown was fifth out at 29, lbw to a Footitt yorker, Sussex had lost four wickets for 18 runs in 7.3 overs.
Captain Ed Joyce, their saviour on a number of occasions this year, looked in the mood for another rescue act before he was lbw to Matt Higginbottom for 17, though he looked unhappy with the decision.
That left Sussex 62 for 6 and recovery was beyond them. Ben Brown pushed forward to Alex Hughes and edged to first slip then after lunch Luke Wright, who scored a vigorous 49 from 75 balls with nine fours, drove at a wide delivery from Groenewald and was caught behind to leave them 124 for 8.
Steve Magoffin had made just 1 when he went back and across and missed a straight ball from Footitt, for a simple lbw decision, and the innings ended when Chris Jordan, who made 23 and had done his best to shield last man Monty Panesar from the strike, attempted to pull a short delivery from Groenewald and lost his leg stump.
When Derbyshire batted again they were given a positive start by Slater and Chesney Hughes, who put on 69 before Hughes was bowled for 34 as he played no stroke to Jordan, giving the bowler his 51st wicket of the season. But Slater went on to score his maiden Championship half-century, and first for Derbyshire in any format, and was well supported by his captain, whose undefeated 62 was his second half-century of the match.