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 late win at Stamford Bridge.
Stephen Ireland was the architect for a magnificent victory, scoring one goal and making another as Villa scored twice in the final seven minutes to stretch Chelsea's winless run to four matches.
Ireland, back in the side for the injured Emile Heskey despite
recent off-pitch antics
, drew the visitors level from close range following Didier Drogba's penalty opener and then teed up a simple third for substitute Darren Bent after Stiliyan Petrov had put Villa ahead on 83 minutes.
Few could argue with the result after another lacklustre display from the Londoners which saw them leave the arena to a chorus of boos and means they drop to fifth spot and out of the Champions League places.
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.
Villas-Boas bemoans 'unfair' result
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.
On the slide
Fifth-placed Chelsea are lower than fourth on 31 December for the first time in 10 years since they were ranked sixth on 31 December 2001
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.
'Brilliant' new year win for McLeish
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 home after a misplaced Lampard pass and great unselfish play from the inspired 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."
Full Time The final whistle is blown by the referee.
Richard Dunne challenges Fernando Torres unfairly and gives away a free kick. Strike on goal comes in from Juan Mata from the free kick.
The assistant referee flags for offside against Darren Bent. Free kick taken by Raul Meireles.
The ball is delivered by Ramires, Stephen Warnock gets a block in.
The ball is crossed by Juan Mata.
Shot on goal by Ramires from long range goes harmlessly over the target.
Stephen Ireland provided the assist for the goal.
85:22 Goal scored
Goal! - Darren Bent - Chelsea 1 - 3 A Villa Darren Bent finds the back of the net with a goal from the edge of the penalty box to the bottom right corner of the goal. Chelsea 1-3 Aston Villa.
Shot by Gary Gardner. Petr Cech makes a save.
Foul by Jose Bosingwa on Gabriel Agbonlahor, free kick awarded. Stephen Warnock takes the direct free kick.
Substitution Barry Bannan replaces Stiliyan Petrov.
Ciaran Clark provided the assist for the goal.
82:06 Goal scored
Goal! - Stiliyan Petrov - Chelsea 1 - 2 A Villa Stiliyan Petrov grabs a goal from inside the box to the bottom right corner of the goal. Chelsea 1-2 Aston Villa.
Jose Bosingwa crosses the ball, clearance made by Richard Dunne.
The ball is delivered by Raul Meireles, clearance by Richard Dunne.
Ashley Cole crosses the ball, clearance made by Stephen Warnock.
Substitution Gary Gardner joins the action as a substitute, replacing Marc Albrighton.
Substitution (Aston Villa) makes a substitution, with Darren Bent coming on for Charles N'Zogbia.
Fernando Torres takes a inswinging corner from the left by-line played to the near post, save by Bradley Guzan. Shot from outside the box by Raul Meireles goes wide right of the target.
Corner taken left-footed by Juan Mata from the right by-line, clearance by Richard Dunne. Raul Meireles takes the inswinging corner, Bradley Guzan makes a save. Fernando Torres takes a shot. Blocked by James Collins.
Jose Bosingwa takes a shot. Save by Bradley Guzan.
Substitution (Chelsea) makes a substitution, with Jose Bosingwa coming on for Paulo Ferreira.
Didier Drogba has an effort at goal from just inside the box that misses to the left of the target.
Foul by Stephen Ireland on Ramires, free kick awarded. Raul Meireles restarts play with the free kick.
Long range effort by Raul Meireles goes harmlessly over the crossbar.
David Luiz delivers the ball, blocked by Ciaran Clark.
The ball is sent over by Raul Meireles, clearance made by Richard Dunne.
The ball is delivered by Raul Meireles, Richard Dunne makes a clearance.
Centre by Didier Drogba, Ciaran Clark makes a clearance.
Juan Mata produces a cross.
Marc Albrighton has an effort at goal from 18 yards. Save made by Petr Cech.
The ball is delivered by Juan Mata, Close range header by John Terry goes over the bar.
Booking Carlos Cuellar goes into the book for unsporting behaviour.
Carlos Cuellar challenges Didier Drogba unfairly and gives away a free kick.
Didier Drogba produces a cross, James Collins manages to make a clearance.
Free kick awarded for a foul by Charles N'Zogbia on Didier Drogba. Direct free kick taken by Raul Meireles.
The ball is crossed by Fernando Torres.
Fernando Torres takes a shot.
Ashley Cole restarts play with the free kick.
Substitution Fernando Torres on for Daniel Sturridge.
Marc Albrighton challenges Daniel Sturridge unfairly and gives away a free kick.
A cross is delivered by Juan Mata, Stephen Warnock manages to make a clearance.
The ball is sent over by Ashley Cole, blocked by James Collins.
Gabriel Agbonlahor has an effort at goal from close range which goes wide right of the target.
Substitution Oriol Romeu leaves the field to be replaced by Frank Lampard.
Free kick awarded for an unfair challenge on Paulo Ferreira by Marc Albrighton. Petr Cech restarts play with the free kick.
Gabriel Agbonlahor takes a shot. Petr Cech makes a save.
Inswinging corner taken right-footed by Raul Meireles played to the near post, James Collins makes a clearance.
Centre by Ashley Cole, Carlos Cuellar gets a block in.
Free kick awarded for a foul by Ramires on Stephen Warnock. Charles N'Zogbia delivers the ball, James Collins produces a shot from close in which goes wide of the right-hand upright.
The ball is swung over by Daniel Sturridge, blocked by Stiliyan Petrov.
The ball is crossed by Charles N'Zogbia.
Juan Mata sends in a cross.
The second half kicks off.
45:00 +2:01 Half time
Half Time The ref blows to signal half-time.
Stephen Warnock concedes a free kick for a foul on Didier Drogba. Juan Mata crosses the ball in from the free kick, Ramires concedes a free kick for a foul on Richard Dunne. Bradley Guzan takes the free kick.
The ball is swung over by Paulo Ferreira, clearance by James Collins.
Corner taken by Juan Mata, James Collins makes a clearance.
Stiliyan Petrov gives away a free kick for an unfair challenge on Oriol Romeu. Shot comes in from Didier Drogba from the free kick.
Unfair challenge on Raul Meireles by Stephen Ireland results in a free kick. Shot comes in from Didier Drogba from the free kick.
The ball is delivered by Ramires, Richard Dunne manages to make a clearance.
A cross is delivered by Paulo Ferreira, clearance by Richard Dunne.
Ramires challenges Charles N'Zogbia unfairly and gives away a free kick. Free kick taken by Stephen Warnock.
Didier Drogba produces a cross, Juan Mata takes a shot. Blocked by Stephen Warnock.
Stephen Ireland produces a volleyed right-footed shot from 18 yards. Petr Cech makes a comfortable save.
Free kick taken by James Collins.
Booking John Terry is cautioned by the ref for unsporting behaviour.
John Terry gives away a free kick for an unfair challenge on Gabriel Agbonlahor.
Juan Mata delivers the ball, Marc Albrighton manages to make a clearance.
The ball is sent over by John Terry, clearance made by Richard Dunne.
Free kick awarded for an unfair challenge on Oriol Romeu by Stiliyan Petrov. Oriol Romeu takes the direct free kick.
The assist for the goal came from Charles N'Zogbia.
27:35 Goal scored
Goal! - Stephen Ireland - Chelsea 1 - 1 A Villa Stephen Ireland fires in a goal from inside the six-yard box to the bottom left corner of the goal. Chelsea 1-1 Aston Villa.
Stephen Ireland takes a shot. John Terry gets a block in.
Charles N'Zogbia takes a outswinging corner from the left by-line played to the near post, Didier Drogba makes a clearance.
Marc Albrighton crosses the ball.
Stephen Warnock sends in a cross, clearance made by John Terry.
22:46 Goal scored
Goal! - Didier Drogba - Chelsea 1 - 0 A Villa Didier Drogba scores a placed penalty. Chelsea 1-0 Aston Villa.
Unfair challenge on Didier Drogba by Richard Dunne results in a Penalty.
Daniel Sturridge produces a cross, Bradley Guzan makes a save.
Shot from just outside the box by Didier Drogba misses to the right of the goal.
Shot from deep inside the area by Daniel Sturridge clears the bar.
Oriol Romeu concedes a free kick for a foul on Stiliyan Petrov. James Collins takes the free kick.
Gabriel Agbonlahor takes a shot. Blocked by David Luiz.
Centre by Ashley Cole, Stephen Warnock makes a clearance.
Effort from just outside the area by Raul Meireles goes over the bar.
Corner from the left by-line taken by Juan Mata, clearance by Richard Dunne.
Centre by Daniel Sturridge, save by Bradley Guzan.
Free kick awarded for a foul by Charles N'Zogbia on John Terry. Direct free kick taken by John Terry.
Juan Mata takes the outswinging corner, clearance made by Stiliyan Petrov.
Outswinging corner taken by Juan Mata from the left by-line, James Collins makes a clearance.
Centre by Juan Mata, Carlos Cuellar gets a block in.
Marc Albrighton crosses the ball, Close range header by Gabriel Agbonlahor goes over the bar.
Foul by David Luiz on Gabriel Agbonlahor, free kick awarded. Stephen Warnock takes the direct free kick.
Gabriel Agbonlahor crosses the ball, Paulo Ferreira manages to make a clearance. Corner taken by Marc Albrighton from the left by-line, Petr Cech makes a save.
Free kick awarded for a foul by Richard Dunne on Ramires. The free kick is delivered left-footed by Juan Mata from right wing.
Corner taken left-footed by Juan Mata from the right by-line, clearance by Richard Dunne.
Juan Mata takes a shot. James Collins gets a block in.
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