Aston Villa left Chelsea's title challenge in tatters and piled the pressure on beleaguered Blues manager Andre Villas-Boas with a clinical display and thoroughly deserved win at Stamford Bridge.
Villa scored twice in the final 10 minutes to ensure the hosts' win less run stretched to four games.
Stephen Ireland was the architect, drawing Villa level from close range after Didier Drogba's penalty opener and then teeing up a simple third for substitute Darren Bent after Stiliyan Petrov had put the visitors ahead.
Few could argue with the result after another lacklustre display from the Londoners, who left the pitch to a chorus of boos.
They had started encouragingly and the ever-impressive Juan Mata narrowly missed out on flicking in an early opener after a good move, while the equally lively Daniel Sturridge also shot over after cutting in from the right.
But the visitors had the measure of Chelsea tactically throughout and their 4-2-3-1 formation worked a treat.
Gabriel Agbonlahor's pace and endless running kept the hosts' centre-backs occupied and allowed man of the match Ireland to find space and time on the ball to feed Charles N'Zogbia and Marc Albrighton on either flank.
The visitors were happy to let Chelsea have possession and looked to play on the break but they still had to do it the hard way after gifting Chelsea the first goal of the match.
Referee Mark Halsey played advantage after an obvious foul 60 yards from Villa's goal and Drogba was felled in the box by a clumsy Richard Dunne challenge.
The Ivorian got up and, with Frank Lampard dropped to the bench, assumed penalty responsibilities to coolly stroke home his 150th goal in a Chelsea shirt.
But the lead only lasted five minutes as Chelsea once again failed to build on their advantage.
Inevitably Ireland was involved, starting a move on halfway and making his way into the box to score on the second attempt after a great run by N'Zogbia.
The hosts responded and Drogba blazed three efforts off target, including two free-kicks, but it was Villa who looked the more likely to score in the middle third of the match.
Villa's patient counter-attacking football and organised rearguard frustrated Chelsea and gaps started to appear in the home defence as they chased a win they so desperately needed to stay in touch with the Premier League leaders.
Ciaran Clark almost gave Villa the lead when the ball bounced off him following a free-kick before the otherwise excellent Agbonlahor wasted a glorious chance, firing straight at Petr Cech when one-on-one.
However, just as in the Boxing Day game against Fulham, Chelsea upped the tempo in the latter stages and created several decent chances.
Drogba dragged a low shot wastefully wide, Jose Bosingwa fired wide from distance and substitute Fernando Torres's miserable run and luck continued when he crashed a shot against the bar moments after coming on.
But Chelsea ultimately paid the price for their failure to deal with Villa's pace up front, and after weathering the storm, the visitors went ahead when Petrov slotted calmly past Cech from Clark's through-ball.
As Chelsea piled forward the gaps at the back grew even bigger, resulting in Bent slotting into an unguarded net after great unselfish play from Ireland.
Chelsea manager Andre Villas-Boas:
"Now we are fifth, below the top four. But we have to wait and see.
"It can change within 48 hours, though. That's a good thing.
"We have a chance to produce something."
He added: "Our reality is a fourth-placed finish, but that is not good enough.
"We have to continue to pursue the best possible finish in the league, and a humble finish for this club."
Villa boss Alex McLeish:
"It's my most satisfying day so far, but I'm hoping there are many more to come.
"I'm a guy who perseveres, who gets back up after being knocked down pretty regularly.
"The players can enjoy this tonight - I don't mean going out on the town, but in terms of confidence levels."