A late Frank Lampard penalty handed Chelsea a vital 2-1 win over English Premier League leaders Manchester United to breath new life into the title race.
Alex Ferguson's side had taken the lead in the first half thanks to a thunderous strike from England forward Wayne Rooney but Brazil defender David Luiz drew Chelsea level.
Then Russia international Yuri Zhirkov drew a foul from Chris Smalling in the 79th minute and Lampard blasted his spot kick high into the net to hand the defending champions a crucial win.
United's misery was compounded in injury time as Serbian defender Nemanja Vidic was sent off after earning a second yellow card and will now miss the trip to Liverpool on Sunday.
The real victors were second-placed Arsenal who can close the gap on United at the top of the table to just one point if they win their game in hand. The Gunners still have to play United at home.
Chelsea's victory keeps alive their faint title hopes and moves them into the Champions League places in fourth but they still trail United by 12 points.
After the game Ferguson took aim at referee Martin Atkinson telling Sky Sports: "It was a bad one to concede the first goal but the penalty kick is so soft, deary me. We played well but we didn't deserve that. That's three years in a row referee's decisions have changed the game."
Ferguson also thought Luiz should have been sent off after he blocked Rooney off the ball when he was already on a yellow card: "It's incredible, he'd done Chicarito [Javier Hernandez] off the ball, late, nothing done, ref's in front of it. He does Rooney clear as day and the ref doesn't do anything. That changes the game."
Carlo Ancelotti admitted Luiz may have been "lucky" but said United were still "too far ahead" for Chelsea to have any realistic chance of retaining their title.
Ancelotti again chose to start $80.5 million striker Fernando Torres ahead of Didier Drogba, who began on the substitutes bench.
Chelsea had the first chance as Florent Malouda connected with Ramires' cross but his effort was right at United's veteran Dutch keeper Edwin Van der Sar.
Just before the half hour mark United broke the deadlock after Rooney found space outside the area and lashed a low shot into the bottom corner of the net.
Chelsea nearly hit back before halftime as Lampard's free kick was parried by Van der Sar but Branislav Ivanovic couldn't convert the rebound.
Eight minutes after the interval Chelsea were level as new signing Luiz popped up at the back post to fire a volley inside the post.
After half chances at both ends the game was settled from the spot after substitute Zhirkov knocked the ball through Smalling's legs and was tripped. Lampard hammered the ball into the roof of the net emphatically.
Two minutes later Zhirkov almost made the game safe as he crashed a volley towards the bottom corner but Vidic managed to divert the ball onto the post and away for a corner.
Fabio almost rescued United with a late run into the box but he couldn't reach the ball to apply a finish and Chelsea held out for a confidence-boosting win.
Elsewhere, Reading, who play in the second tier of English football, knocked Premier League Everton out of the FA Cup.
Matt Mills struck the only goal as he pounced on a rebound to slam a volley into the net on 26 minutes.
Reading were thankful to a stunning late save from Alex McCarthy who denied Leon Osman and ensured his side booked a quarterfinal clash with either Manchester City or Aston Villa, who play their fifth round replay on Wednesday.
