Tottenham stunned Chelsea to blow the Premier League title race wide open with a second derby win in four days. Manchester United's earlier victory at rivals City had pulled them to within a point of leaders Chelsea - but the Blues succumbed to an inspired Spurs.
Jermain Defoe opened the scoring from the spot after John Terry's handball and Gareth Bale slammed in a second.
Terry saw red for two bookings before Frank Lampard's late reply, but Spurs held on to move above City into fourth. It means with three games to go, including a tricky trip to Liverpool on 2 May, Chelsea remain top of the table but with United breathing closely down their neck and with Ars*nal six points behind but with a game in hand.
Blues boss Carlo Ancelotti will also be without captain Terry for one of those games through suspension following a red card few can argue with after two poor tackles.
As for Tottenham, they now have a two-point cushion over fifth-place Manchester City following a couple of wins that not only give their push for a Champions League spot a huge fillip but also delighted a buzzing White Hart Lane crowd.
Manchester United, chasing an historic fourth successive Premier League crown, got the favour from Spurs they had hoped for, having beaten City in the lunchtime kick-off before Harry Redknapp's men took three points from Chelsea.
But even the most optimistic of United fans will have failed to have foreseen a Spurs performance that confounded those who thought the north Londoners would run out of steam on the back of a jam-packed week.
This was an impressive display with Bale superb on the left, Luka Modric controlling in the middle, and Michael Dawson a rock at the back.
And, just as he did in the derby win over Ars*nal on Wednesday, goalkeeper Heurelho Gomes once again came up with the goods on the odd occasion he was called upon.
Not that that was very often as the home side came up against an out-of-sorts Chelsea side.
Florent Malouda forced a smart save from Gomes just seven minutes in with a 25-yard stinger, but otherwise it was the hosts who enjoyed the better of the chances.
Roman Pavlyuchenko and Bale had already had snap-shots either saved or cleared when the latter went down in the box under a forceful challenge from Jon Obi Mikel.
Referee Phil Dowd ruled the Chelsea man had got the ball, but the decision went in Tottenham's favour just a minute later when Terry handled Bentley's cross.
Defoe, who had missed six of his last 10 penalties, made no such error this time, drilling an unstoppable spot-kick into the corner with Petr Cech diving the wrong way.
The visitors responded briefly when Didier Drogba's half-chance was easily saved, but at the other end Pavlyuchenko forced Cech to tip over the bar from range before David Bentley was denied a second from six yards out by Terry's outstretched leg.
The west Londoners thought they had levelled when Malouda slid home from Yuri Zhikov's pass, but the Frenchman was correctly ruled offside.
And then, six minutes later, Tottenham doubled their lead, Bale cutting inside full-back Paulo Ferreira and slamming in at the near post with his weaker right foot.
Redknapp's men then had their keeper to thank for the fact they went in at the break two goals to the good, after Gomes spectacularly beat away Lampard's fierce volley from 16 yards out.
Ancelotti responded by making a double half-time substitution - Nicolas Anelka and Branislav Ivanovic coming on for Joe Cole and the tormented Ferreira - to add to his earlier replacement of the injured Mikel with Michael Ballack.
However, at the restart Drogba appeared to double up with pain in his groin, prompting the Blues to appeal to change their substitution. Dowd refused, though, and the game continued with the Ivorian looking far from fully fit.
Immediately, it could - and arguably should - have been three, Defoe sent through on goal by Pavlyuchenko's pass but Cech, standing tall, producing a fine stop to tip the England man's shot wide.
Still, Chelsea's frustrations were only heightened when Terry saw red for two bookings in three minutes, the second a clear trip on Bale that the visiting captain can have few complaints about.
From then on in, Tottenham could have scored a hatful. Cech tipped a brilliant Bentley lob over, saved at his near post from Bale, before watching the hosts waste a succession of chances on the break as Chelsea pressed late on - Pavlyuchenko and Bale, again, guilty of firing wide when well placed.
In reply, Chelsea enjoyed some possession but few chances - only really looking a threat after Lampard had slid Ballack's cross home in injury time.
It was far too little, too late, though, and Tottenham deservedly held on for a precious win that will be as gratefully received in Manchester as on both sides of north London.