Our unbeaten FA Cup run continues thanks to a second-half fight back at Old Trafford.
We have not lost a game in this competition in open play since this quarter-final stage five years ago, and thanks to second-half goals from Eden Hazard and Ramires, we live to fight another day in 2013 after a terrible start away to the Premier League leaders.
United went two up inside 10 minutes, Javier Hernandez and Wayne Rooney putting them in control, and only rarely in a tame first half did Chelsea threaten to make it a contest.
Frank Lampard and Demba Ba both brought saves from David De Gea, but the home side looked more likely to add to their tally and we had Petr Cech to thank for keeping us in the tie after a string of important saves.
After the break it was a different story as Chelsea wrestled the initiative back from our hosts and pulled one back through a beauty from sub Hazard before Ramires swept home on the break to level the scores 23 minutes from time.
It set up a grandstand finish in which either side could win it, United through Robin van Persie and Chelsea through Juan Mata, but in the end there could be no winner and so our amazing season goes on with yet another fixture to add to the calendar.
Ba, cup-tied on Thursday, was back in the side as one of seven changes from our defeat to Steaua Bucharest, preferred to Fernando Torres in attack.
John Terry, Branislav Ivanovic and Ryan Bertrand all dropped to the bench with David Luiz the only surviving defender in the 11. Frank Lampard was captain alongside Ramires in deep midfield, with Victor Moses and Juan Mata back in with Oscar behind the striker.
United left Robin van Persie on the bench following their midweek Champions League exit to Real Madrid, as Javier Hernandez started with Wayne Rooney up front.
Underdogs as we sought to defend our FA Cup crown, the one thing Chelsea did not want to do was concede early, so there would have been despair in the dugout to see United gifted two early goals within 10 minutes of the start.
With the first meaningful attack of the game, United went ahead, owing to a rare mistake from Cech. Michael Carrick sent a long pass from midfield into the Chelsea area, and with the goalkeeper hesitating, Hernandez was able to peel off Gary Cahill and send a looping header into the empty net.
Before the Blues had time to respond, the home side had doubled their lead. A swift counter attack ended with Nani being tripped by Moses on the United left. Rooney bent in the free-kick which evaded everybody, bouncing in front of Cech and into the far corner.
Man U v Chelsea
Chelsea's first chance to get back into the game came on 19 minutes, but it came purely through luck, with Rafael misplacing a dangerous square ball straight to Moses, who saw his shot blocked by the legs of a defender without working David De Gea in the United goal.
Ba and Mata then linked up inside the United half, the latter squaring for Lampard, seeking his 200th Chelsea goal, to bend into the goalkeeper's arms.
United were soon back on top though, and threatened to go further ahead with another typical fast counter, Nani crossing low for Hernandez who arrived before Azpilicueta but flicked his shot wide of the near post.
Cech then had to save excellently twice, first from a low Rooney shot, and then from a David Luiz header after the defender headed the rebound back towards goal. From the resultant corner Tom Cleverley eventually hit the target again with a vicious drive, but Cech held on.
Man United v Chelsea
There was more work for De Gea from Ba as the Blues improved, but United were still the more convincing side, and with a two-goal lead, they did not need to take any unnecessary risks. Carrick was rarely caught ahead of the ball, and when it did bypass him, Jonny Evans and Rio Ferdinand were well placed to clear.
Chelsea's best move of the half involved Mata and Moses, the Spaniard back-heeling for the Nigerian inside the area, but he then miscued his shot way, way wide when a much-needed goal looked likely. Azpilicueta then beat Shinji Kagawa but could not find a team-mate with his cross, and when United countered, the full-back was cautioned for a trip on Hernandez.
It had not been a good half.
Rafael Benitez made no substitutions at the break, but within six minutes of the restart was making a double switch. Lampard and Moses came off, John Mikel Obi and Hazard their replacements. Just previously Ramires had burst forward and shot wide.
Chelsea now had a greater urgency and United were spending more and more time on the back foot, but a breakthrough still looked unlikely.
Then, Hazard found a yard of space on the edge of the United area, and bent a wicked effort into the far corner with De Gea stranded. Game on.
Man U v Chelsea
Nine second-half minutes passed with little in the way of action, United replacing Hernandez with Van Persie, and neither side looking like scoring.
Chelsea won possession down the left, the ball finding its way to Oscar inside the United half, and after taking a touch he played it to Ramires on his right. With Evans between him and the goal, the Brazilian had plenty to do, but turned his marker one way then the other to find space to shoot and then calmly slotted the ball low into the far corner. It was a quite superb equaliser.
Old Trafford was stunned, United were wounded. How would they react?
Van Persie almost fired them back ahead but twice saw efforts blocked, and then the Dutchman opted to go alone on the counter instead of playing in fellow sub Danny Welbeck, but could not find his way through the Chelsea defence.
Mata brought another save from De Gea and sub Torres won us a fruitless corner as thoughts of a replay began to enter the mind, when such a game could go ahead is anyone's guess.
Thirty seconds before the end of normal time it all looked academic, Mata bringing down David Luiz's pass, turning Evans and shooting low from six yards. Nine times out of 10 it would be a goal, but this time De Gea stuck out a leg, as he does so well, and diverted it around the post.
There was still time after for Hazard to sting the keeper's palms, but he would not be beaten again.
From being dead and buried, we had been prevented a last-minute winner at the home of the league leaders. We might not have won, but Chelsea, simply, do not lose in the FA Cup.
Manchester United (4-4-2): De Gea; Rafael, Ferdinand, Evans, Evra (c); Nani (Valencia 45+1), Carrick, Cleverley, Kagawa (Welbeck 75); Rooney, Hernandez (Van Persie 62).
Unused subs Amos, Vidic, Anderson, Young.
Goals Hernandez 4, Rooney 10
Chelsea (4-2-3-1): Cech; Azpilicueta, Cahill, David Luiz, Cole; Ramires, Lampard (c) (Mikel 52); Moses (Hazard 52), Mata, Oscar; Ba (Torres 77).
Unused subs Turnbull, Terry, Ivanovic, Bertrand.
Goals Hazard 58, Ramires 67
Booked Azpilicueta 41, David Luiz 54, Hazard 75
Referee Howard Webb