Second-half goals from Eden Hazard and Ramires ensured that Chelsea remain in the FA Cup as they staged a second-half comeback to cancel out strikes from Manchester United duo Wayne Rooney and Chicharito.
The first half was dominated by the Red Devils as Chelsea endured a torrid first 10 minutes, with Chicharito opening the scoring with a header, before Rooney extended the lead with a fortuitous free-kick.
It was truly a tale of two halves, though, as Chelsea controlled the second period, eventually getting their just rewards with a stunning effort from Hazard and a smart finish from Ramires.
Sir Alex Ferguson chose to retain the bulk of the side that fell to a disappointing Champions League exit on Tuesday, though he made four changes as Jonny Evans, Shinji Kagawa, Rooney and Chicharito came in, at the expense of Nemanja Vidic, Danny Welbeck, Ryan Giggs and Robin van Persie.
Rafa Benitez, meanwhile, made wholesale changes as only Petr Cech, David Luiz, Oscar and Frank Lampard remained from the side that fell to a 1-0 defeat against Steaua Bucharest on Thursday, with John Terry, Hazard and Fernando Torres amongst those notably excluded.
It took just four minutes for the deadlock to be broken and it came by virtue of a Chicharito header as the home side took an early lead. Michael Carrick played a magnificent through-ball into the diminutive Mexican's path and the striker took it early on his head, surprising the advancing Cech by looping the ball deftly over his head and into the far corner.
With Chelsea still shellshocked, the Red Devils managed to extend their lead mere minutes later. After Victor Moses felled Nani out wide following a typically rapid counter-attack, Rooney stepped up to swing in the free-kick. Despite seemingly attempting to find a teammate with an inswinging cross, no-one managed to get a touch on the ball and it found its way past Cech and into the far corner, leaving the Blues chasing a two-goal deficit with just 10 minutes gone.
Chelsea continued to struggle as the half wore on as both Chicharito and Rooney spurned chances to extend their lead further, with the away side's opportunities in the first period limited to half-chances; Moses should have done better after cutting inside in the area, instead slicing his shot well wide, while David De Gea was called into action after a dangerous Juan Mata cross.
Though Manchester United's impetus continued into the second half, Chelsea began to come into the game and it took 15 minutes and a moment of brilliance for their pressure to show. Hazard, after receiving the ball to the left of the penalty area, took one step inside and curled the ball into the far corner with a great deal of finesse, leaving De Gea with no chance.
With 20 minutes remaining, United attempted to put themselves out of distance with a third goal and, in doing so, committed too many men forward and Ramires was the men that made them pay.
Luiz intercepted an attack on the edge of his area and set the counter-attack in motion, which eventually ended with Ramires bearing down on goal and, after the Brazilian turned onto his left, he stroked the ball into the corner, levelling proceedings.
Chelsea had the better of the last 10 minutes but were unable to capitalise on their pressure as Mata had the best chance to win the game, though De Gea was on hand to produce an excellent save with his right boot, meaning that a replay beckons.