Tottenham confirmed the shift in north London's balance of power by beating Arsenal 2-1 in the Premier League on Sunday, opening up a seven-point gap between the rivals in the fight for Champions League qualification.
Near-identical goals by Gareth Bale — his 10th in his past eight games — and fellow winger Aaron Lennon in a three-minute span at the end of the first half proved decisive for Spurs, whose euphoric post-match celebrations gave an indication of the psychological importance of the victory. It left them third with 10 games remaining.
Per Mertesacker's 51st-minute header that deflected in off Bale gave Arsenal hope of a third straight derby comeback from 2-0 down in a typically absorbing and frantic match at White Hart Lane. But the visitors couldn't avoid a costly loss that leaves them fifth, denting their chances of playing in the Champions League for a 16th straight season under Arsene Wenger.
"It's not a big enough margin yet to be completely safe," said Tottenham manager Andre Villas-Boas, whose team is now unbeaten in 12 league matches. "But what we think is that it can have a direct effect on their motivation — and on ours, too."
With Arsenal likely to go without silverware for an eighth successive season, Wenger has been pinning his hopes on a top-four finish to salvage something from a miserable campaign. But even that is in serious doubt now, with Chelsea five points ahead in fourth place. There is no more room for error — and such is Arsenal's fragility, especially in defence, that further setbacks seem likely between now and the end of the season.
"It's a massive challenge," Wenger said. "I still think it's possible and will fight for it, that's for sure."
Arsenal came back from a 10-point deficit to Spurs last season to secure qualification for Europe's top competition on the final day, but Wenger had Robin van Persie up front then. And Bale wasn't in the kind of scintillating form that is currently earning him comparisons with Lionel Messi and Cristiano Ronaldo.
The Wales winger is virtually carrying Tottenham back into the Champions League on his own, with his latest strike buttressing his strong chances of winning English football's Player of the Year award. It also ensured he equalled the club record — held jointly by Robbie Keane, Rafael van der Vaart and Teddy Sheringham — of scoring in five straight Premier League games.
Playing centrally rather than on the left wing, he had arguably been having his quietest match of the season up until his goal, with Arsenal starting the stronger and its oft-criticized defence looking compact.
It then simply disintegrated.
First, an unchallenged Gylfi Sigurdsson slipped a ball through for Bale to beat the offside trap in a central position before finishing with the outside of his left foot past goalkeeper Wojciech Szczesny. His 20th goal of the season in all competitions was met with chants of "He scores when he wants" by the delirious home fans.
Arsenal's back line, which has been suspect all season especially through the middle, hadn't learned its lesson because Scott Parker threaded a pass behind the docile Thomas Vermaelen and Lennon rounded Szczesny to slot into an empty net only 140 seconds later.
It was extremely disheartening for Wenger, whose team made had its best start to a big game for some time.
Only a last-gasp tackle by Spurs defender Jan Vertonghen, the game's outstanding player, stopped Olivier Giroud from going through on goal, while the impish Jack Wilshere — playing in a more attacking role — looked more than capable of picking off Tottenham's high defensive line. The team's movement and passing had been sharp and crisp but it was all undone in a flash.
"It's difficult to understand how we lost a game like that," Wenger said.
Arsenal came from 2-0 behind to win the last two north London derbies 5-2, and its fans dared to dream of another comeback when Mertesacker rose highest to glance in a free kick from Theo Walcott, the header taking a touch off Bale. Both of Mertesacker's goals this season have come against Spurs.
More goals have been scored in north London derbies than in any other head-to-head rivalries in the Premier League, so it was no surprise to see them flying in once again.
There should have been more, in Arsenal's net, but Bale skied over from close range, Sigurdsson chose to pass rather than shoot when one-on-one with Szczesny, and then Jermain Defoe — on for the injured Adebayor — dribbled a shot wide.
Substitute Tomas Rosicky gave Arsenal more urgency in the final 20 minutes but Spurs goalkeeper Hugo Lloris wasn't seriously troubled, with Vertonghen and Michael Dawson having mighty games at centre half in front of him.
Such is Tottenham's form is that the team may yet overtake second-place Manchester City, which is only two points clear ahead of Monday's match at Aston Villa. Finishing fourth and ahead of Arsenal will be Spurs' priority, however.
"The difference from last year is that we are on an upward spiral in terms of confidence and they are on a negative spiral in terms of results," Villas-Boas said. "To come out of that negative spiral will be very difficult."
Villas-Boas said Adebayor and Mousa Dembele, who both went off injured, in the second half should be fit for Thursday's Europa League last-16 match against Inter Milan.