Swansea City surrendered all three points at Pride Park as their promotion push suffered a setback.
The Swans were the victims of their own downfall after a bizarre own goal from Ashley Williams gave Derby a seventh minute lead.
Despite dominating long periods of the game, Stephen Davies fired the Rams two goals ahead after the break before the Swans staged a late fight-back when Darren Pratley pulled a goal back.
While Swansea deserved at least a share of the spoils, they came away with nothing with nine Championship games left to play.
Swans boss Brendan Rodgers kept faith with the same team that drew 1-1 with Watford at the Liberty Stadium on Tuesday.
The draw was enough to keep the Swans in second place, but after the previous defeat at Scunthorpe the visitors were looking for maximum points from this afternoon's encounter.
Goalkeeper Dorus de Vries marshalled a back-four that saw Angel Rangel and Neil Taylor at full-back and skipper Alan Tate partner Ashley Williams in the heart of the defence.
Scott Sinclair and Nathan Dyer remained out wide with Joe Allen, Mark Gower and Stephen Dobbie in the central midfield roles.
Luke Moore returned to the club he spent three months on loan before joining the Swans on a permanent basis in January. The 25-year-old continued as the central striker.
Garry Monk, Andrea Orlandi and Tom Butler all travelled to Pride Park after returning from injury, but the Swansea boss resisted throwing them onto the bench.
They faced a Derby side struggling in 19th place after a barren run that has seen them go without a win at home since November.
Swans:Dorus de Vries, Angel Rangel, Neil Taylor, Alan Tate (c), Ashley Williams, Joe Allen, Mark Gower, Stephen Dobbie, Scott Sinclair, Luke Moore, Nathan Dyer. Subs: Yves Ma-Kalambay, Leon Britton, Craig Beattie, Darren Pratley, Albert Serran, Jazz Richards, Cedric van der Gun.
Derby: Frank Fielding, John Brayford, Shaun Barker, Daniel Ayala, Gareth Roberts, Paul Green, Robbie Savage, Stephen Pearson, Alberto Bueno, Steven Davies, Jamie Ward. Subs: Dean Leacock, Theo Robinson, Miles Addison, Conor Doyle, Callum Ball, Jeff Hendrick, Matt Duke.
Ref: G D Scott
Att: 25,341Away: 1,190
Swansea made a great start as they forced two early corners. Mark Gower took them and when his second effort found his way back to him, he delivered a great cross to the far post which the stretching Ashley Williams headed just over.
The visitors remained on the offensive as Nathan Dyer looked dangerous down the right; creating a fifth minute chance for Gower who blazed high into the stand.
But disaster struck in the sixth minute when Ashley Williams played a back pass to Dorus de Vries. But the 'keeper was over the other side of his goal and by the time de Vries got across the ball just crossed the goalline as Derby took the lead in comical fashion.
The large Swans following behind the goal looked as stunned as their team.
While Swansea got their heads up and re-grouped, the goal had given Derby a lift and Angel Rangel was harshly booked for a foul on Stephen Pearson with 12 minutes gone.
Williams went some way to making amends in the 20th minute when he cleared the ball off the head of Jamie Ward as he prepared to grab a second at the far post.
And it was Williams who nearly grabbed the equaliser two minutes later when he leapt above the Derby defence at the far post to head a Gower cross against the bar.
Swansea were slowly starting to get on the ball again as Dobbie tried his luck from the edge of the area which Frank Fielding saved with ease.
Dobbie came even closer in the 35th minute with a fierce effort from 25 yards which flew inches the wrong side of the post.
All the chances were falling to the Scot as he did well to create space for himself just inside the area; flashing a shot wide when he should have hit the target.
Derby pushed for the second just before the break as de Vries had to get down well to save a dangerous ball across the six-yard area by Ward.
Half-time: Derby 1 Swans 0
Swansea made two changes at the start of the second-half with Leon Britton and Darren Pratley replacing Gower and Moore.
It meant Dobbie pushed forward into the centre-forward's role.
It was an encouraging start as Britton had a powerful shot blocked before Dobbie fired wide in quick succession.
Derby made their first substitution in the 49th minute when Conor Doyle replaced Paul Green.
It didn't stop Swansea driving forward as Dobbie sent Dyer through, but he delayed his shot and Derby cleared their lines.
Joe Allen forced a good save out of Fielding with a shot from the edge of the area, before Derby nearly caught Swansea on the counter-attack when Dyer lost the ball on the edge of the Rams area. But Britton was back to clear the danger.
The relief was short-lived, however, as Derby grabbed their second in the 54th minute when Doyle's corner was headed in by Stephen Davies.
It was substitute Darren Pratley who opened up the game again though as he arrived at the far post to head home an Angel Rangel cross to make it 2-1 with 66 minutes gone.
Swans boss Brendan Rodgers immediately brought on Craig Beattie for Dyer as the visitors sensed another comeback of Middlesbrough proportions.
And the equaliser nearly came three minutes later when Dobbie cut inside before his deflected shot hit the cross bar.
Almost immediately Sinclair was racing through under pressure from Shaun Barker, but the winger could only toe-poke his shot just wide.
Derby made another change with Theo Robinson coming on for Davies with 20 minutes remaining.
Swansea were piling on the pressure in the final stages, but were leaving them exposed at the back as Williams was harshly booked for a foul on Robinson on the halfway line.
And when Tate lost the ball to Alberto Bueno in the 80th minute, the game should have been over. But substitute Robinson blazed a glorious chance into the stand.
In the end time ran out for the Swans.