It's too much of a cliche that this was a game of two halves, Chelsea being rubbish in the first but the better in the second - but in this case it's true.
What should be explained is why Chelsea were so bad, a goal down in 35 minutes and then another straight after the start of the second to gift Manchester City two goals from which it was impossible to recover from.
Only with defeat staring them in the face did the Londoners fire into life, too little, too late and Chelsea are out of the FA Cup at the semi-final stage at Wembley.
A hopeless first half performance saw them concede a 35 minute goal from Samir Nasri.
In truth, the light Blues could and should have been out of sight by the time the second half resumed.
Within seconds of that restart, Chelsea were caught with a sucker-punch goal from Sergio Aguero.
Only then, did they remember that they were in fact the cup holders and that absolute embarrassment was just a kick away.
Fernando Torres entered the fray on 65 minutes and suddenly the dark Blues found their feet and scored a classic from Ba.
From then on it was City defending like Trojans with the occasional counter-attack.
But it was not to be, Torres had half a claim for a penalty turned down and Chelsea found reserve goalkeeper Costel Pantilimon in inspired form.
This defeat will not go down well with the fans nor with John Terry and Frank Lampard, plonked in the bench,with not a sniff of the action.
The opening exchanges of the game were to be expected -cagy but City were the first to settle to create a goal-scoring chance.
Yaya Toure broke free to slide a perfect slide ball to Sergio Aguero, on left. The Argentina striker outpaced Cesar Azpilicueta and struck a fine shot, which luckily for Petr Cech hit his side netting.
Two minutes later and the Chelsea goalkeeper produced a brilliant low, to his right, reflex save, again from Aguero.
This was quickly followed by a near-post save by Cech from a Carlos Tevez blaster as the Londoners felt the full onslaught from the side one place above them in the league.
Branislav Ivanovic was, on 20 minutes, quick to throw himself in front of a Tevez shot, after the Argentine striker beat the offside trap from a long ball from Vincent Kompany.
Chelsea finally had an attack worthy of the name on 24 minutes. From a Juan Mata free kick. The ball was punched out by City goalkeeper Costel Pantilimon straight to Eden Hazard, 20 yards from goal. The midfielder kicked the ball into the ground, it bounced over the defence and goalkeeper but Kompany was on hand, on the goal line to head the ball to safety.
Gareth Barry was the first to enter referee Christopher Foy's book when a yellow card was brandished for hacking down Ramires two minutes later.
The Blue half of Manchester finally got their just rewards for their dominance with 10 minutes of the half remaining.
Toure strode through the Chelsea midfield like a juggernaut. Just outside the Chelsea penalty area, he fed the ball to Agureo which was knocked onto Samir Nasri. The French midfielder got slightly lucky to see his touch rebound off Azpilicueta back to him and he rammed a fierce right-footer, past Cech from six yards out.
Toure received a yellow card of the stroke of half time for tripping up Mikel.
Chelsea arrived back onto the pitch early, perhaps signalling that they mean business.
Within seconds of the restart, Ramires entered Foy's book for a foul on James Milner.
Quite what happened to Chelsea's defence after that is a mystery as a simple chipped ball on 47 minutes, from Barry, saw the Chelsea's rearguard open like the Parting of the Red Sea to allow Aguero free rein to caress a simple header past a hopelessly exposed Cech to put City fans into dreamland.
Such was City's complete control of proceedings, that Kompany, on the hour, even had the confidence and temerity to try his hand at a 35-yarder, which to be fair to him was not that far away from goal.
In a seemingly forlorn attempt to reverse Chelsea's fortunes, interim boss Rafael Benitez brought £50m Fernando Torres from off the bench to replace midfielder Mikel on 65 minutes.
It resulted in a change of formation with a twin strike attack of Torres and Ba, albeit, with the Spain striker operating more on the left
Thirty seconds later and Chelsea were back in the semi-final tie. David Luiz lumped the ball forward, Kompany misjudged the flight of the ball and Ba was on hand to scoop the ball past Pantilimon. From close range.
With Chelsea's tails up they almost equalised two minutes later when Mata thought he was upended by the Pantilimon, but in fact the goalie did well to dispossess the Spain midfielder.
A minute later and after brilliant set-up play by Hazard on the byline, Ba unleashed a venomous shot from close range which Pantilimon snaffled to his chest.
A David Luiz blistering free-kick on 79 minute inched wide of City's goal as Chelsea forced the pace in a desperate attempt to rekindle their FA Cup hopes.
Torres thought he had a penalty after racing through on goal with four minutes to go. Kompany certainly had a pull on his shirt. Unfortunately it was out of the eyeline of referee Foy.
With the seconds ticking down, Chelsea threw everything at their opponents but each time the City defence held firm.
The Londoners last real effort came from the wayward right boot of Hazard who blasted a 30-yarder high and wide.
City deserve their place in the final, back at Wembley to face Wigan Athletic.
For Chelsea, why they did not play until they were 2-0 down is anybodies guess. it cost them dear, it cost them their FA Cup.
Teams: Chelsea: Cech, Ivanovic, Luiz, Ramires, Mata, Oscar, Obi, Hazard, Azpilicutea,
Ba, Bertrand.
Subs, Turnbull, Lampard, Torres, Moses, Terry, Benayoun, Ake.
City: Pantilimon, Zabaleta, Kompany, Clichy, Nastasic, Toure, Milner, Nasri, Barry, Tevez, Aguero
Subs: Hart, Kolo Toure, Lescott, Kolarov, Garcia, Sinclair, Dzeko
Ref: Chris Foy