Derby boosted their play-off hopes by dealing a big blow to Birmingham's own chances with a 1-0 win at Pride Park.
Birmingham went into the game knowing time was running out to try and get in the top six but they were second best throughout to a Derby side who thoroughly deserved a fourth successive home win.
The game was decided towards the end of the first half when Birmingham keeper James Beadle misjudged a Rhian Brewster header although Derby could have won by a bigger margin.
Birmingham were very poor and never looked like equalising let alone winning against a lively and inventive team managed by John Eustace who they sacked in October 2023.
Derby's bright start matched the early spring sunshine and after Birmingham gave the ball away, Brewster fired narrowly wide from just outside the area.
Birmingham were being pressed into mistakes and Patrick Agyemang got in front of Christoph Klarer in the 10th minute but could not lift the ball over Beadle.
All the pressure was coming from the home side in a game Birmingham needed to win and it was no surprise when they went ahead in the 43rd minute.
Derry Murkin sent the ball over and Brewster's downward header beat Beadle who thought it was going wide and reacted too late to prevent it squeezing inside his left post.
It left Birmingham with a lot to do in the second half but Derby took up where they left off with Beadle getting an unconvincing punch on a free-kick before Matt Clark headed just over from a corner.
Sam Szmodics forced Beadle into a diving save after Derby again got in behind Birmingham and it was no surprise when the visitors made a triple substitution in the 59th minute.
There was little for the Birmingham fans to cheer and it took until the 65th minute for their team to have a shot at goal with Jonathan Panzo firing well wide.
Joe Ward almost added a second with a low drive that struck the outside of a post with Birmingham showing no sign of getting back into the match.
They had posed hardly any threat in the final third while Derby looked a danger whenever they went forward with Agyemang having a shot blocked before Bobby Clark brought another save from Beadle.
Birmingham huffed and puffed but apart from some speculative crosses into the box, did not remotely look like denying Derby a richly deserved victory.
The managers
Derby's John Eustace:
"It was a fantastic performance from start to finish although I'm disappointed we didn't get more goals.
"We were really dominant with or without the ball and I thought it was a top performance against a good team who are chasing the play offs.
"We showed no fear in our football which is great and we certainly showed a really exciting attacking brand of football at times so I'm delighted with the effort and really proud of the football club today. We go about our business in the right way and to get the three points is really pleasing.
"It comes with consistency to be able to play the same team week in and week out. Everyone is getting the connection now, we've signed some terrific Championship players with the experience and know-how of the league which is key and we're in a good place going into the final seven games."
Birmingham's Chris Davies:
"Very disappointed with the result and performance, we didn't deserve anything from the match.
"It's very frustrating and the story of our season in many ways. If that game was at St Andrews it would look completely different, we would have been on the front foot the entire game but away from home we've had many performances like this.
"The strength and personality to really hurt the opposition hasn't been there and ultimately it's my responsibility, that's my job to get that out of the players and put a team on the pitch that can do that.
"Today was a big, big game for our season and it's even more painful that we lose the game in such a fashion."
