In control throughout the first half, Derby were quick out of the traps in the second half as an impressive Tom Ince broke the deadlock at the AESSEAL New York Stadium.
Darren Wassall’s side continued to dominate and added two further goals in the same amount of time as Chris Martin and Ince made it 3-0, before the unlikeliest of comebacks began to unravel.
Danny Ward’s first seemed a mere consolation before Leon Best stepped off the substitutes bench to score his first goals in nearly two years to stretch their unbeaten run to four games.
Before Wassall's side settled into the game, it took as little as five minutes for the in-form hosts to register their first shot on target in an energetic start to the contest after pinching back possession deep inside their own half.
The turnover from there took two passes as Danny Ward was sent clear through the middle, but the former Huddersfield Town man was denied brilliantly by Scott Carson.
From there, the Rams had to hold firm from the resulting corner, before Lee Camp was then called upon to ensure did not break the deadlock.
Ince looked lively from the first whistle and forced a fine save from the former Rams goalkeeper after stepping inside from the right and pulling the trigger after evading his nearest challenger.
The Rams grew in strength and created all the notable chances at the AESSEAL New York Stadium – growing in stature as the contest wore on.
On 18 minutes, with Derby showing more control in midfield and looking far more comfortable in possession, Bradley Johnson cut across a loose ball with a first time shot that whistled narrowly wide of Camp’s upright.
Jeff Hendrick, like Johnson restored to the starting XI, shot narrowly wide from the edge of the box and after the meeting descended into one of its quieter spells, Richard Keogh, the Derby Captain, watched a looping header drop on to the roof of the net from a deeper corner.
With two minutes of added time shown, Camp was forced into his best save of the afternoon as he got down low to get a firm hand to Martin’s low shot after being picked out just inside the 18-yard box by Ince.
Derby’s wideman was showing just what he was capable of against the Millers and continued to show his quality in the early stages of the second half.
The 24-year-old teed up Johnson early on with a cross, but the midfielder could only fire over Camp’s crossbar, but the large travelling contingent of Derby supporters did not have to wait much longer before their side opened the scoring.
After Craig Bryson’s shot from then edge of the area was charged down, Marcus Olsson dinked the ball back into the area, a cross which was only half cleared by the Rotherham defence.
It proved costly as Ince took control on the edge of the area before sweeping a low left shot beyond the reach of Camp to send the Rams faithful into raptures.
It should have been two moments later after Cyrus Christie was sent down the right side and put in a teasing centre towards Martin, but the in-form forward, stretching, could not divert the ball in at the far post.
Again, he did not have to wait much longer before he added his name to the score sheet as he finished some neat play between Hendrick and Christie, finishing the latter’s cross.
Then, within seconds, came the third as Martin charged down a weak back pass from the kick off and beat Camp to the ball, leaving Ince the simple job of placing the ball in the back of the unguarded net.
Wassall’s side remained on the front foot, showing as much control in a game as they had done all season, as Martin and Ince both forced Camp into routine saves, whilst Bryson shot over from range with little under 10 minutes remaining.
Then the unlikely began to unfold as Rotherham fought back to set up a grandstand finish at the AESSEAL New York Stadium.
Their first goal, what could only look as a consolation, came following a stray pass from Keogh. Grant Ward seized on possession and the ball was quickly turned over to Danny Ward in acres of space to finish off the inside of the far post.
Then came the second. Warnock’s side quickly pressed for a corner from the replay from which Best, a former Derby loanee, capitalised on some weak defending to bundle the ball in at the far post – the forward’s first goal for nearly two years.
With four minutes of added time shown, the unlikely did happen. Rotherham again pressed high up, Danny Ward showed intelligence to flick the ball over the head of his nearest challenger and cross to the back post.
Derby did not deal with the dangerous ball in and there was Best to head beyond Carson to complete the unlikeliest of comebacks. This was football at its most bizarre and inexplicable.
Rotherham United: Camp; Kelly, Broadfoot, Collins, Mattock; Green (Best, 69), Smallwood; Derbyshire (Newell, 69), Frecklington, Thomas (G Ward, 31); D Ward
Substitutes not used: Kenny, Doyley, Burke, Clarke-Harris
Derby County: Carson, Christie, Keogh, Shackell, Olsson; Thorne (Blackman, 75), Hendrick, Bryson; Ince, Johnson, Martin (Bent, 75)
Substitutes not used: Grant, Buxton, Butterfield, Weimann, Hanson
Attendance: 11,163 (2,621 Derby supporters)