Two strikes from Lucas Joao saw Sheffield Wednesday beat promotion-chasing Derby 2-0 at Hillsborough.
The victory was manager Jos Luhukay's first league win since taking charge, putting a temporary halt on their drop towards the relegation zone.
The result saw Derby fail to put the pressure on second-placed Aston Villa and leave them with only one win from their last five games.
Joao tucked away the first goal clinically, then scored with a stunning solo effort two minutes into the second half. The travelling team applied the pressure early on, but the Owls defended resiliently to cling onto just their fourth home clean sheet of the season.
Wednesday, who were seeking back-to-back home wins over Derby for the first time since 1987, made seven changes from Saturday's 1-1 draw at Barnsley, as Luhukay continued the search for his first three points.
The changes seemed to have an impact, as the Owls started brightly and 21-year-old Sean Clare, who impressed on his first league start, flashed a shot wide before Frederico Venancio nodded a corner straight at 'keeper Scott Carson.
The home side then took the lead after a swift move, when Adam Reach lofted a pass into the box towards Jack Hunt. Hunt calmly laid the ball off to Joao (18) who slotted the ball into the bottom right corner with his first touch.
Andreas Weimann wasted Derby's best chance of the half, when he headed Chris Baird's cross narrowly wide of the front post.
Hillsborough erupted just after the break, when Joao (47) doubled his side's advantage from 25 yards. The Portuguese striker drove inside unchallenged and unleashed a powerful driven shot into the bottom corner from distance.
The visitors, having not lost a league game since November, responded strongly. Bradley Johnson first had a penalty appeal waved away, after going down under pressure from Clare, then David Nugent headed high and wide before Tom Huddlestone saw his ambitious first-time shot blocked in the area.
Weimann had another golden chance to put Derby on the scoresheet, as Matej Vydra rolled in behind, but Wednesday stopper Joe Wildsmith stood strong and thumped the stinging effort out for a corner.
Sky Bet Championship's top goal-scorer, Vydra, was next to be thwarted by Wildsmith. Almost inside the six-yard box, the striker looked set to score but Wildsmith managed to keep him at bay with his legs.
The Rams continued to waste chances as Nugent selflessly released Johnson into the box and he scuffed his shot well wide. Weimann nearly got one back at the end.