Derby dealt a devastating blow to Newcastle's title aspirations on a night when Chris Hughton's side lost top spot in the Coca-Cola Championship following a 3-0 loss.
The Magpies arrived at Pride Park on a club-record 15-game unbeaten run but they were outclassed and outfought by a team that has breathed new life into its relegation fight.
Derby, still on the crest of a wave from their victory over rivals Nottingham Forest, were rewarded for their first-half dominance when Rob Hulse headed them in front five minutes before the break.
Kris Commons doubled the advantage from the penalty spot before the hour mark and Shaun Barker completed the rout soon afterwards.
It was Newcastle's first league defeat since losing 2-1 at Scunthorpe on October 20, and West Brom's home win over the Iron saw them replace the Magpies at the summit.
Derby started brightly with Stephen Pearson causing problems down the left.
The Scot twice found space inside the opening five minutes.
Hulse was inches away from connecting with a low, teasing cross as he stretched in front of goal before Commons headed another centre straight at Stephen Harper from six yards out.
It took Newcastle until the half-hour mark to threaten Stephen Bywater's goal, and the promotion hopefuls were given more than a helping hand.
Nicky Hunt gave the ball away to Jonas Gutierrez midway inside his own half - it was the second time the on-loan Bolton right-back had casually gifted the Argentina international possession in a dangerous area.
Gutierrez got to the byline and his cross was headed narrowly past the near post by Derby defender Barker.
County captain Robbie Savage escaped with just a yellow card for a typically robust challenge on Wayne Routledge after 37 minutes.
The former Wales international let the ball run away from him and always looked certain to catch Routledge late as the winger beat him to the ball.
Three minutes later Derby deservedly went ahead when Hulse rose unmarked inside the six-yard box to head home Commons pinpoint cross.
But Newcastle responded well and created two good chances in the space of as many minutes before the break.
Kevin Nolan's through-ball sent Routledge clear down the right side of the box but his driven shot was blocked by the advancing Bywater.
The Derby keeper then pushed Nolan's volley behind at his near post after the Toon skipper burst into the area.
Newcastle carried the momentum into the second half and had the ball in the net inside a minute of the restart.
Andy Carroll headed home Routledge's cross but referee Anthony Taylor had already blown for a foul on the winger.
Hunt then bravely blocked a goalbound effort from his former Bolton team-mate Nolan.
However, Derby survived the storm and extended their lead in the 59th minute.
Pearson's through-ball sent Michael Tonge clear. The on-loan Stoke midfielder had taken the ball too wide but he was bundled over by Gutierrez inside the area.
Commons coolly sent Harper the wrong way from the spot.
Four minutes later, Newcastle failed to clear a corner and when the ball was lifted back into the area, Barker turned it in from six yards.
Hughton immediately brought on Shola Ameobi, Peter Lovenkrands and Alan Smith but his side failed to break down a dogged Derby defence.
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