Chelsea's lead at the top of the Premier League is down to only two points after they were crushed 3-0 at home by an inspired Sunderland.
Superb goals from Nedum Onuoha and Asamoah Gyan, together with a late third from Danny Wellbeck, were enough to give Steve Bruce's side a deserved victory - their first at Stamford Bridge in a decade.
The result moves Sunderland up to seventh and leaves Chelsea only a couple of points above the Gunners, having spurned the opportunity to go five clear in the title race.
Needless to say the defeat is a bitter blow for the Blues, whose preparations for the match were ruined when captain John Terry had to pull out with sciatica.
With fellow centre-half Alex also missing it left full-backs Paulo Ferreira and Branislav Ivanovic to form a makeshift partnership at the centre of the Chelsea defence - and Sunderland took full advantage.
Black Cats manager Steve Bruce made the brave decision to play two up front against the champions and was rewarded with a fizzing first-half display from his team.
Chelsea goalkeeper Petr Cech had to be at his very best to make a fine double save from Gyan and Kieran Richardson - and the Czech international also kept out Wellbeck when he was clean through.
So when Sunderland grabbed an opening goal on the stroke of half-time it was no real surprise - although the manner of the goal could not have been expected.
With both Wellbeck and Gyan causing constant problems for Chelsea's makeshift defence, Chelsea failed to pay attention when right-back Nedum Onuoha picked up the ball on the left of the penalty area.
That proved to be a big mistake because the English defender, on loan from Manchester City, slalomed his way spectacularly through the Blues defence before cutting a shot across Cech and into the net.
Chelsea, who had created few clear cut chances themselves, were astonished - and the situation got worse after the break.
Sunderland, buoyed by their success, made it 2-0 when Jordan Henderson - only just called up to the England squad - slipped a pass through for Gyan to score in the 53rd minute.
Finally Chelsea were roused into action, but they found Sunderland as gritty in defence as they were energetic in attack and could not find a breakthrough.
In fact, Wellbeck got the goal his performance so deserved when he poached a late goal from Ashley Cole's woeful back-pass in the 88th minute. That summed up both Sunderland's energy - and Chelsea's miserable defensive display.Meanwhile Arsenal moved to second in the Premier League with a 2-1 win at Goodison Park but had to survive a spirited fightback from Everton.
Arsene Wenger's side took a stranglehold on the game with strikes from Bacary Sagna and Cesc Fabregas and they looked like they might repeat last season's 6-1 mauling.
But Arsenal took the foot off the gas and Tim Cahill's late strike meant it was a furious finish.
The win keeps the pressure on leaders Chelsea but it was a disappointing result for David Moyes' men, who now languish in lower midtable.