Billy Godleman hit a career-best 77, but Derbyshire lost to Northants Steelbacks by seven wickets at The 3aaa County Ground.
The Falcons’ chances of reaching a quarter-final were dealt a major blow as Josh Cobb blasted 73 not out off 37 to power the visitors to victory after Godleman and Wayne Madsen (38) had earlier helped Derbyshire to 177-6.
Derbyshire remain fifth in the Vitality Blast North Group on 10 points, with three games left to play.
Calum MacLeod edged to slip and was out for four in the third over after Derbyshire won the toss and elected to bat.
In at three, Anuj Dal soon followed suit and was bowled by Nathan Buck after mis-judging a back of a length delivery.
Godleman remained the constant and looked to drive and pull the seamers as the Falcons ended the powerplay on 41-2.
Powerful hitter Wahab Riaz clubbed a big six down the ground, but was run out by the next ball for 15 and Derbyshire stuttered slightly before reaching 70-3 at the halfway stage.
It was at this point that Godleman and Madsen, set and accustomed to the conditions, began to score more freely with fours around the ground, including a trademark scoop over the wicketkeeper from Madsen,
The pair took Derbyshire past 100 shortly before Godleman reached his third half-century of the campaign, off 40 balls with three fours and one six.
The opener doubled his six tally the following ball as a bottom-hand punch cleared the ropes and the stand, but Madsen was out with the score 138-4 and the hosts were pegged back slightly.
Gary Wilson (24 not out) looked to pick up where Madsen left off as Godleman powered his way to 77 before he was run out to leave the hosts 169-5 with six balls to go.
A good final over from Nathan Buck restricted the Falcons to 177-6, to leave the game finely balanced at the halfway stage.
Ben Duckett got the Steelbacks off to a flyer in what proved to be an entertaining chase which shifted in momentum throughout.
The opener hit 41 off 29 but was bowled by Alex Hughes, who claimed his 15th of the campaign, and Ricardo Vasconcelos saw his stumps clattered by Lockie Ferguson for 25 as Derbyshire clawed their way back.
The visitors were 90-2 at the after ten overs and 20 ahead of where Derbyshire were at the same stage. With a platform set, Cobb, who came into the game on the back of 103 against Birmingham Bears, scored quickly and took the game away from the Falcons.
He reached his half-century off 27 balls, which included five sixes, as the Steelbacks measured their chase well to leave them needing 31 from the final five overs.
Alex Wakely batted nicely alongside the fluent Cobb and the pair’s unbroken 80-run stand was the decisive partnership as Cobb hit seven sixes in his match-winning innings and the Steelbacks ran out seven-wicket winners with 10 balls remaining.
The Falcons travel to Blackfinch New Road tomorrow night to face Worcestershire Rapids, knowing a win would keep their hopes of reaching the knockout stages alive.
Vitality Blast is back! Derbyshire Falcons are back at home on Saturday 11 August against Leicestershire Foxes. Purchase your tickets in advance and save from £4. Buy online or call 01332 388 101.