Derby recorded their biggest win of the season as a 4-0 triumph at Birmingham saw them climb out of the bottom three for the first time since the end of October.
Three goals in 10 first-half minutes did the damage for Wayne Rooney's side as they responded strongly to their stoppage-time defeat by Preston on Boxing Day, which had ended a previous six-game unbeaten run.
It also did no harm to Rooney's chances of being appointed as the permanent successor to Phillip Cocu, who was sacked as manager in early November, as the club await a potential takeover by a consortium.
However, Birmingham are now looking over their shoulders as Aitor Karanka's men have not won any of their last five and their last home victory was back on October 28.
The Rams had started the night as the Sky Bet Championship's lowest scorers with 10 in 20 matches and they had not scored more than two in any game, but they put that right within 25 minutes.
They could have gone in front after 10 minutes when Louie Sibley – in for the suspended Martyn Waghorn – was played in on goal, but Neil Etheridge stuck out a strong hand to push the effort over the bar.
But the goalkeeper was the villain in the 15th minute as he gifted the visitors the lead. Etheridge made two howlers in his last game at St Andrew's and here was another as he dropped Graeme Shinnie's corner under no pressure.
Krystian Bielik was the grateful recipient as he hooked the ball into the empty net for his first Derby goal.
And Etheridge was beaten again in the 17th minute. Blues captain Harlee Dean challenged Sibley and referee Tony Harrington pointed to the spot, with Shinnie emphatically converting the penalty.
Derby were rampant and Etheridge made a fine save to prevent Sibley from making it three.
But the reprieve was temporary as Rooney's side took advantage of some dreadful defending from Shinnie's resulting corner.
Matthew Clarke flicked it on at the near post and everyone in blue watched as Colin Kazim-Richards slid it in unchallenged at the back post.
The one-way traffic continued as Sibley had one effort deflected into the side netting and Clarke had a header kicked off the line by Adam Clayton.
Karanka made four changes to his Birmingham side at half-time in search of a response, sending on Lukas Jutkiewicz, Jonathan Leko, Jake Clarke-Salter and Mikel San Jose, but it made little difference.
And Jason Knight added to their woes in the 77th minute, converting Kamil Jozwiak's cross as Derby scored four away from home for the first time since their famous play-off semi-final win at Marcelo Bielsa's Leeds in May 2019.
