Brede Hangeland inflicted a significant early dent in Arsenal's title hopes as his goal earned Fulham a surprise 1-0 win at Craven Cottage.
The Norwegian defender struck midway through the first half to hand Roy Hodgson's side a deserved triumph and give Arsenal manager Arsene Wenger much to ponder. The Frenchman's side looked flaky and uninspired against their London neighbours, who avoided relegation by a hair's breadth last season, and the Frenchman is now likely to step up his search for new talent before the transfer window closes on 1 September.
A new midfielder remains Wenger's top priority - he saw his team over-run in the middle at Craven Cottage - but few of his charges should feel assured of their place after this limp effort. Wenger, who has amassed his fair share of silverware, knows that titles are never won while the leaves are still on the trees, but they can be lost.
His side proved as much two seasons ago when a stuttering start left them out of contention by Christmas but fate appeared to be smiling on the Frenchman this season. A kindly fixture list, which sees Arsenal dodge all the top flight big-hitters until November, when Manchester United travel to the Emirates, represented an open invitation for a team of such talent to stock-pile points before the onset of winter.
But every step on a title challenge is potentially treacherous. Arsenal had laboured to a scrappy victory over newly-promoted West Bromwich Albion last week and the lack of key personnel at Craven Cottage - including Cesc Fabregas and Tomas Rosicky - ensured this London derby had a more even feel to it.
That impression was confirmed in a feisty first half, where Fulham gave as good as they got, snapping hungrily into every tackle and harrying their illustrious guests out of that smooth style which can be so destructive. Fulham's industry did not translate into a welter of chances, but when they did carve out an opportunity, they were ruthless.
In the 21st minute, Jimmy Bullard's corner was met flush on the volley by the onrushing Hangeland and the ball crashed into the bottom left-hand corner. Arsenal were ruffled, with even their most seasoned performers suffering.
Robin van Persie had missed a gilt-edged early chance, smashing wide from 10 yards after a neat set-up from Samir Nasri, and there was further frustration when Emmanuel Adebayor's towering header in the 23rd minute smacked the right-hand post. However, there was little else to concern Mark Schwarzer in the Fulham goal and Wenger's slumped shoulders and furrowed brow as he trooped off for half-time told their own story.
There was at least a renewed purpose and urgency to Arsenal's football after the interval, but the cutting edge remained conspicuous by its absence. Instead, the most likely scorers were Fulham, with Bobby Zamora flashing over at the near post after a surging run down the left from South Korean forward Seol Ki-Hyeon.
Arsenal upped the tempo in the dying stages in a desperate bid to restore parity and Van Persie almost succeeded when his low drive zipped just wide. However, there was to be no dramatic rescue act and Wenger knows that unless he acts fast in the transfer market, this could be another season of broken dreams.