David de Gea made a terrific stop in the dying seconds to deny Chelsea a famous win after Rafael Benitez's side came roaring back to earn a 2-2 draw with Manchester United in the FA Cup sixth round at Old Trafford.
United had moved two goals clear and apparently on the cusp of a spot in the semi-finals when Javier Hernandez headed home on five minutes with Wayne Rooney - restored to Sir Alex Ferguson's starting line-up after his omission from the Champions League defeat to Real Madrid - finding the net with a free-kick six minutes later as Chelsea's defence was again found wanting.
Chelsea were rocking, but the mood of the match changed when Benitez introduced Eden Hazard for Victor Moses on 52 minutes.
The visiting side suddenly began to revel in large dollops of possession and with Juan Mata playing at his very peak, Chelsea restored parity courtesy of exceptional goals by Hazard (59) and Ramires (68).
The outgoing Champions League holders probably deserved the win and would have departed Manchester with it if De Gea hadn't thrown out a boot to deflect a Mata shot wide after his fellow Spaniard had left Jonny Evans for dead deep inside the United box.
The semi-final draw was not overly kind to either side. The winner of this match face fellow Premier League heavyweights Manchester City in the last four at Wembley next month while Wigan meet Blackburn or Millwall, who drew 0-0 earlier in the day and replay on Wednesday night.
There is no fixed date pencilled in for the Chelsea-United replay at Stamford Bridge.
Chelsea were trying to gather their senses when they fell behind early on as Hernandez nodded over Petr Cech from Michael Carrick's delightful ball over the top.
United moved two to the good when Rooney's free-kick somehow eluded a crowded box before nestling in the corner of the net as Chelsea were made to pay for some more slipshod defending.
Mata showed his class for the first time when he set up Frank Lampard on 23 minutes, but the England midfielder could only scoop a shot straight into the arms of De Gea.
United teetered on the brink of a third goal before the break as Hernandez flicked Nani's cross wide with Cech producing two magnificent stops to save from Rooney and then prevent David Luiz from scoring a headed own goal as he swiped the ball over his bar.
United's prospects were not helped when the injured Nani hobbled out of the action to be replaced by Antonio Valencia seconds before half-time, but Chelsea were a different proposition in the second period and appeared to come to life when John Obi Mikel replaced Lampard with Hazard taking over from Moses.
Mata played provider for Hazard to net from just inside the United box as the Belgian took one touch before curling the ball high into the net beyond De Gea.
With United being forced further and further back, Chelsea equalised when Luiz broke up a home attack before Chelsea bounded to the other end. Demba Ba released Ramires on the run and he planted a crisp shot beyond the diving De Gea.
Chelsea could easily have won it in the death throes of the match, but De Gea's goal would not be breached for a third time as the keeper made another save from a blistering Hazard hit moments after he had somehow prevented Mata from sinking the winner.
Rio Ferdinand was also fortunate to avoid a red card late in the match when he clearly felled substitute Fernando Torres off the ball that would surely have prompted stern action by referee Howard Webb if he had witnessed the home defender's shenanigans. Ferdinand, like the rest of the United side, cut a frustrated figure as Chelsea mounted their recovery.
MAN OF THE MATCH
Juan Mata (Chelsea): An exceptional performance by the little Spanish midfielder, who probably deserved to be on the winning side such was his appetite to help his side recover.
MAN UTD: De Gea 8, Rafael 7, Ferdinand 7, Evans 7, Evra 7, Carrick 6, Cleverley 6, Nani 5, Rooney 7, Kagawa 6, Hernandez 6. Subs: Van Persie 6, Valencia 6, Welbeck 6.
CHELSEA: Cech 7, Azpilicueta 6, Luiz 8, Cahill 7, Cole 6, Ramires 8, Lampard 5, Mata 9, Oscar 7, Moses 5, Ba 7. Subs: Mikel 7, Hazard 8, Torres 6.