Yohan Benalouane's early goal settled the East Midlands derby as Nottingham Forest prevailed 1-0 at the City Ground.
The defender found the back of the net against Derby after just two minutes, and Frank Lampard's side failed to find a way back into the Sky Bet Championship clash on Monday night.
Victory for Derby would have seen them climb back into the top six, but they stay seventh in the table. Forest, meanwhile, who are just a place below their fierce rivals, are now also just a point behind them and three off the play-offs.
Both sides made one change. Ben Watson came in for Nottingham Forest for the injured Lewis Grabban, while Andy King started for Derby in place of Craig Bryson in midfield.
The first East Midlands derby of the season at Pride Park finished goalless, but on this occasion Forest took the lead almost instantly.
Joe Lolley swung a free-kick in that Jaden Bogle failed to defend properly. Daryl Murphy then nodded the ball on and Benalouane popped up at the far post to poke home.
Derby's only real chance of the first half came after 14 minutes when Duane Holmes slipped Martyn Waghorn in behind, but his attempt to dink Costel Pantilimon saw the ball go wide.
Forest had a couple of decent chances themselves to double their lead. Two minutes after Waghorn went close, a Fikayo Tomori mistake allowed Lolley the opportunity to play in Murphy, but his first touch let him down and denied him a clear sight of goal. Then after 34 minutes Jack Robinson found Lolley in the box, but his effort was diverted just wide by Ryan Yates.
Nottingham Forest: Pantilimon (7), Darikwa (7), Benalouane (8), Milosevic (7), Robinson (7), Watson (6), Yates (7), Colback (8), Lolley (7), Osborn (6), Murphy (7)
Subs: Guedioura (n/a), Bonatini (n/a), Wague (n/a)
Derby: Roos (6), Bogle (7), Tomori (6), Keogh (6), Cole (6), Holmes (6), Huddlestone (5), King (5), Wilson (5), Waghorn (4), Marriott (4)
Subs: Jozefzoon (6), Johnson (n/a), Bennett (n/a)
Man of the match: Jack Colback
Lampard made a change at half-time as Tom Huddlestone made way for Florian Jozefzoon, but it didn't make any real difference to their performance and Forest eventually secured their first derby win since 2015.
Man of the match - Jack Colback
It was a game of relatively few chances at either end and that was mainly due to the superlative midfield work done by Colback and Forest. They kept Derby almost entirely quiet throughout the game.
Martin O'Neill: "We showed a great deal of determination. Sometimes you can set out tactics and have a lot of plans but eventually it was phenomenal determination that won the game and carried us through. We can improve and we know this ourselves, but Derby are a fine side and I'm delighted we won for the players and fans.
"The atmosphere was fantastic and the song beforehand was reminiscent [of old times]. The willpower of the crowd to keep us going towards the end when we were getting tired was fantastic."
Frank Lampard: "On reflection I don't think it was a fantastic game quality-wise. When you give a goal away from a set-piece that floats around the box in the first minute it plays into Nottingham Forest's hands. They are a very organised team who can counter-attack against you, but on the other hand we didn't do enough to draw or win the game.
"It's disappointing, and we have to fight out of this little rut we've been in for the last few games. We had loads of control of the ball but it was just in the final third where we are a bit short at the minute. It's hard to create and we need to work hard on the training ground to get that back."
Both sides are back in Sky Bet Championship action at 3pm on Saturday. Derby head to Aston Villa and Nottingham Forest visit Stoke.