Posh arrived at Pride Park as the division's form team having set a club record by going 10 games without defeat at this level but they struggled to live with the Rams until home goalkeeper Frank Fielding was sent off in the 62nd minute.
Substitute Grant McCann scored the resulting penalty after Fielding's dismissal but Darren Ferguson's side had fallen behind to goals from Craig Bryson, Chris Martin and Richard Keogh either side of half-time, and only goal difference keeps Posh outside of the relegation zone.
Former Posh playmaker Paul Coutts' mishit cross that smashed against a post after two minutes put the visitors on the back foot and that is where they remained for the entire first half.
Derby's first-half domination was rewarded shortly before the break when Jamie Ward and Connor Sammon combined down the left before the latter picked out Bryson in the box who blasted home from close range.
Ferguson made two changes at half-time, but had conceded two more goals within the first seven second-half minutes.
First, Martin doubled the lead with a superbly-taken low shot on the turn and then Keogh headed in the third goal after being left unmarked at the far post to nod in Craig Forsyth's flick-on.
The Rams suffered a setback just after the hour mark when Fielding brought down Dwight Gayle who was clean through.
He received his marching orders and McCann sent his penalty past substitute goalkeeper Adam Legzdins.
It gave Posh fresh impetus through Lee Tomlin, who shot wide, and then Gayle's drive was blocked by Legzdins. But Ferguson's side could not score again and face a nervy final two games.
- Sports & Recreation
- Darren Ferguson
- Frank Fielding
- Craig Bryson
- Chris Martin
- Richard Keogh