The evergreen Merson gave the Rams a lesson in possession as he and Nigel Quashie ultimately embarrassed their hosts with the quality of their passing.
But Pompey were indebted to the pair as they wrestled control of a match that had gone against them in the early stages.
Derby, buoyed by back-to-back victories over the teams of Sheffield bristled with intent at the outset, but once they fell behind not even the re-introduction of Fabrizio Ravanelli could halt the leaders.
They was a tangible sense of optimism in Pride Park at kick-off and that was well-founded when the Rams opening the scoring after 16 minutes. Lassina Diabate's clumsy challenge on Lee Morris resulted in a penalty, which Danny Higginbotham drove into the roof of the net.
Midfielder Rob Lee then took it upon himself to try and double the lead with a trio of long-rangers which sadly for his club failed to trouble Shaka Hislop in goal.
Portsmouth had only mustered one shot on goal when they drew level on 27 minutes. Todorov started the move by sliding a pass down the inside left channel towards Burchill, who was tackled by Higginbotham.
But the ball broke to Matthew Taylor who crossed for Todorov to volley home from 12 yards.
Portsmouth took the lead six minutes into the second half as they turned defence into attack with all the speed of Premiership champions Arsenal.
Paul Boertien lost the ball deep inside the Portsmouth half and Pompey broke in a blur of passing between Steve Stone, Quashie and Merson.
Within seconds the ball had been fed to Taylor and his perfect cross found Burchill unattended to guide a header into the bottom corner.
Todorov then had a goal ruled out for offside and Merson was denied a blatant penalty when chopped down by Lee.
But Derby claimed they had a cast-iron penalty when sub Deon Burton had a fierce goalbound shot apparently deflected away via a defender's arm.
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