It was a breathless encounter, played at relentless pace, and when it was all done Tottenham had a seven-point advantage over their great rivals and the encouragement to believe they can finish above them for the first time in the Arsène Wenger era. They will know from bitter experience how quickly it can change but there are good reasons why Arsenal are involved in a game of catch-up and this defeat, once again, provided the hard evidence.

For long spells Arsenal had the edge, playing with the greater sense of order in a match of frenetic, lung-splitting pace. They subjected Spurs to some prolonged pressure after Per Mertesacker's header six minutes into the second half and gave everything in search of an equaliser.

Yet they lost because of that startling two-minute spell when they went to pieces in defence and an old deficiency was exposed that can make it feel like a trick of the mind that the team began this season with three clean sheets and all that huff and puff about the impact of Steve Bould's appointment as assistant manager.

What a deception it has turned out to be. Arsenal's defending has undermined them in big matches all season, just as it has ever since we started talking about them winning trophies in the past tense. It is a problem that has been neglected for far too long and in those moments, when Gareth Bale and Aaron Lennon scored in quick succession, so much was revealed about the vulnerability of this team.

At the final whistle there were jubilant scenes. Spurs, once again, feels like a happy place and, though André Villas-Boas was correct to preach caution, it felt like a potentially pivotal moment in the modern-day rivalry of these two clubs. The acoustics at White Hart Lane were superb and it finished with the victorious players throwing their shirts into the crowd.

Tottenham have had eight different managers since they last finished above Arsenal, under Gerry Francis in 1995, and finally they appear to have found one who can do it. They are now 12 matches unbeaten in the league, on their best run for 28 years. Bale has 10 goals in eight games and there is a steel about Spurs that does not seem to be there with Arsenal. The home defence were pinned back in the final exchanges but refused to wilt.

Mertesacker finished the match playing as an auxiliary centre-forward, with the Arsenal goalkeeper, Wojciech Szczesny, also suddenly appearing in the opposition penalty area, but Hugo Lloris was well protected in the main.

It boils down to the fact that the home side's centre-halves, Michael Dawson and Jan Vertonghen, were superb, whereas Mertesacker and Thomas Vermaelen made telling mistakes in those moments when Arsenal defended with all the mobility of a row of stalagmites. A team cannot be this generous at the back and hope to get away with it.

For Arsenal, it can already be held up as Exhibit A why, barring a football miracle, they are going out of the Champions League. Yet it is also the fundamental reason why they are threatening to finish outside the qualification places for Europe's top competition for the first time in 15 years. If they could defend, this would be some team. Unfortunately for them, they cannot and it led to mocking chants here as the Spurs fans implored Wenger to stay on.

For both Spurs goals it had been one pass that dissected the entire defence, first from Gylfi Sigurdsson and then Scott Parker, but it could never have happened without the worst form of ball-watching. Vermaelen and Mertesacker ushered in Bale to put the first past Szczesny with the outside of his left boot. The left-back, Nacho Monreal, started following Lennon for the second, then inexplicably gave up and Vermaelen, again, showed absolutely no anticipation. Lennon took the ball round Szczesny before sliding his shot into an exposed net and Arsenal's goalkeeper had every right to come sprinting out of his penalty area to berate his team-mates. As straight lines go, Arsenal's defence for those two minutes was more like an infant's squiggle.

It meant they suddenly found themselves 2-0 down when, as Wenger said afterwards, they had not faced any concerted pressure and might even have led by the same margin. Unfortunately for them, Olivier Giroud is not a more prolific striker and, for all the clever probing of Santi Cazorla and Jack Wilshere, there was often no cutting edge. Yet it was disconcerting, too, to see Wenger admit he was at a loss about his team's habit of conceding soft goals in big matches. There was no explanation, or remedy, from a man who once had all the answers, just an acceptance that it was destabilising the whole team and then a quick exit.

Mertesacker's header, from Theo Walcott's free-kick, set up a pulsating second half, taking a slight yet decisive deflection off Bale. There was still the sense, however, that Spurs might catch their opponents on the counter-attack. Sigurdsson had the chance to soothe the crowd's nerves after 69 minutes, played in by the substitute Jermain Defoe, but inexplicably declined the opportunity to shoot when in other moments of the match he was happy to fire in shots from 20 or 30 yards. In the end, it did not matter and Spurs celebrated like a team in no mood to be caught.

Man of the match Jan Vertonghen (Tottenham)