The visitors were bracing themselves for humiliation when two down and a man worse off following the captain's moment of madness but the £50m man came good at the death to triumphFernando Torres' stoppage-time strike sealed an incredible come-from-behind win for 10-man Chelsea, holding on against the odds to dump Barcelona out of the Champions League and reach the final, with John Terry sent off in the first half.
An astonishing end to the first period saw Sergio Busquets open the scoring for the hosts from close range before captain Terry was dismissed for kneeing Alexis Sanchez in the back off the ball.
Andres Iniesta then put the hosts clear before Ramires ran through on the counterattack to chip Victor Valdes and claim a priceless away goal.
Didier Drogba gave away a penalty minutes after the break but Messi uncharacteristically cracked it against the crossbar - and Chelsea held on until it fell to substitute Torres in added time to confirm their place in the final, after rounding Victor Valdes.
Pep Guardiola surprised some by benching Dani Alves, opting for three at the back with Gerard Pique recalled. Youngster Isaac Cuenca was also drafted into the front four.
Roberto Di Matteo, meanwhile, selected the same side that beat Barca 1-0 at Stamford Bridge, with Drogba leading the line once again.
Barcelona started ominously, Messi and Alexis exchanging a quick-fire interchange of several short passes to smuggle the Argentine through into the area, but his effort flew into the side netting.
Chelsea’s troubles started early as centre-back Gary Cahill slipped in the box, clutching his hamstring, and had to be replaced by Jose Bosingwa after just 12 minutes.
Pique was then in the wars, colliding with his own goalkeeper and requiring medical treatment after landing on his head, but he was able to get back up again.
After an opening 19 minutes dominated more by injuries than chances, Cech was called into action. More scintillating neat passes on the edge of the box again put Messi through, this time centrally, but his finish lacked any venom.
Drogba caused brief panic moments later after bypassing Pique in the Barcelona area, though the angle was too tight to trouble Valdes.
That signalled Pique’s last contribution as he succumbed to his earlier knock with Alves replacing the defender one right of a back three.
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After several minutes of frustration the hosts broke through. As Chelsea failed to clear a corner, Cuenca picked the ball up on the left and, with Terry playing several men onside, Busquets was on hand to turn in a simple finish.
Terry's evening got a lot worse, though, as moments after the goal, while standing off the ball alongside Alexis, he appeared to knee the Chilean in the back. The referee, Cuneyt Cakir, produced the red card after prompting from his assistant.
Chelsea’s despair was compounded just before half-time. Iniesta infiltrated forward outside the last defender to receive Messi’s through-ball and dispatched an expert finish calmly past Cech.
But out of nowhere, the visitors got back in it. Ramires, who was booked in the aftermath of Iniesta’s goal and would play no part in the final, latched on to Frank Lampard’s pass on the end of a powerful run, and the Brazilian delivered a thrilling chip over Valdes to rekindle the Blues’ hopes.
The second half started disastrously, though. Drogba, back to help his depleted side out, mistimed his tackle on Fabregas, gifting Barca a penalty. Messi stepped up, ready for his first goal against Chelsea - but crashed his effort onto the crossbar.
Barca continued pushing, as they had to, and Cech was alert when young Cuenca tested him with a shot from the left, closing him down to snuff out the danger.
There might have been a game-deciding second for Chelsea in the 67th minute as substitute Salomon Kalou battled forward on his own, getting past his markers before coming over as a rabbit in the headlights at the crucial moment, allowing Valdes to scoop up.
As the bookings mounted up - Lampard, Messi and Ivanovic, the latter also missing the final - Barcelona's chances dried up. Busquets, going for an unlikely double, was only able to scuff awkwardly over.
The home fans cheered when Alexis had the ball in the net on 81 minutes but substitute Fernando Torres was spared blame for losing his man as it was ruled out for an offside in the build-up.
Just moments later, too, Messi came achingly close, firing against the post from a snapshot as Chelsea held on for dear life, Cech's touch key in flicking it onto the woodwork.
As Barca threw their last at Chelsea, they were undone. With nobody in their own half, £50 million man Torres suddenly found himself clean through with space to burn. Racing forward and rounding Valdes with his most composed head on, the Spaniard rolled it into an empty net to seal a famous triumph for Di Matteo's men.
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