Sussex Sharks' Clydesdale Bank 40 challenge suffered a setback as they lost to Derbyshire Falcons by six wickets.
A top-order collapse saw the Sharks bowled out for 167 in 37.2 overs, with seamer Tim Groenewald taking 3-32, and although the Falcons lost Australian batsman Usman Khawaja to the opening ball of their innings, skipper Wayne Madsen's first limited-overs fifty of the season and Dan Redfern's 49 sealed victory.
Madsen was unbeaten on 64 out of the Falcons' 170-4 as the Sharks tasted defeat in the competition for the first time to stay in second place, a point behind leaders Warwickshire who they play at Hove next week.
The Sharks started well after they were put in but went off the rails in the face of some tight spin bowling from Wes Durston and David Wainwright, who celebrated a four-year contract and his Derbyshire cap by conceding only 26 runs from his eight overs.
Durston went for just 37 and picked up the wickets of Ed Joyce, lbw sweeping, and Matt Machan who was stumped trying to break the shackles after scoring 20 from 32 balls.
Murray Goodwin was involved in two needless run-outs which accounted for Mike Yardy and Joe Gatting in the space of three balls in the 19th over and when Ben Brown sliced a lofted drive at Wainwright to mid-off, the Sharks had lost four wickets in as many overs to slide to 95-6.
Goodwin and Kirk Wernars restored some order by adding 50 but it took them 12 overs and Groenewald returned to beat Goodwin's attempted drive and blow away the tail as the Sharks were dismissed in the 38th over.
The Falcons' chase started atrociously when Khawaja was run out without facing and his opening partner Chesney Hughes followed in the sixth over when he was pinned in front by Amjad Khan.
Durston played some handsome cover drives to score 30 off 35 balls before he was caught behind down the leg side with the score on 55, and another wicket at that stage would have given the Sharks a chance of defending their modest total.
But the Falcons knew they only had to bat sensibly to win the game and Madsen and Redfern did just that to take the game away from the visitors.
With no danger of any interruption from the weather, the pair were content to work the ball into the gaps although Madsen drove Monty Panesar over long-on for the only six of the match in the 28th over before sweeping the England spinner for four.
He reached his fifty off 87 balls and Redfern, who was also awarded his cap before the game, looked set to make his first one-day half-century of the summer but was stumped off Chris Nash which ended a stand of 96 in 20 overs.
But the wicket came too late for Sussex and fittingly it was Madsen who drove Yardy for the winning boundary to complete Derbyshire's fourth victory in Group C.