Eden Hazard's late, late show earned us a draw against a spirited Sparta side, enough to send us into the last 16 of the Europa League.
After a strong start, Chelsea were caught with a blow few had predicted, David Lafata firing the visiting side ahead 16 minutes in, but with chances continuing to come, it looked as though Chelsea would still progress with little trouble.
But chance after chance went begging, for Fernando Torres and Victor Moses in particular, and suddenly Sparta looked like they might nick it on the break as youngster Vaclav Kadlec twice broke clear, but in injury time, substitute Hazard stepped up and earned the necessary draw to avoid extra-time, cutting inside and blasting home to the relief of a full Stamford Bridge.
Rafael Benitez made five changes to the side which started against Brentford. Full-backs Cesar Azpilicueta and Ryan Bertrand returned, with John Mikel Obi back after Nigeria's African Cup of Nations success alongside Ramires in central midfield. Demba Ba, cup-tied, was replaced by Torres up front.
Chelsea, already defending a one-goal lead from the first leg, almost doubled that advantage within five minutes when Torres released Moses down the right flank, and received the return pass on the edge of the box, just dragging his effort wide of the far post with Sparta's goalkeeper Tomas Vaclik at full stretch and looking beaten.
Oscar saw an effort deflected wide before at the other end it took a flying Azpilicueta block to deny Sparta striker David Lafata a goal when he reacted quickest to a loose ball dropping inside the Chelsea box.
Torres then lifted a Juan Mata cross onto the roof of the net from close range with Sparta's centre-backs split, and later headed a corner from his compatriot over the bar.
Chances were not sparse, but the sooner Chelsea could start taking them the sooner we could think about the next round. Sparta would soon show how it was done, and ensure a nervy night ahead in the process.
Once again the ball dropped loose inside the Chelsea area, and once again the visiting player reacted quickest, and when the ball was pulled back to Lafata this time he made no mistake, lashing it high into the roof of the net to draw his side level on aggregate.
Tomas Prikryl brought a near-post save from Petr Cech on the half-hour, and from the resulting corner, Marek Matejovsky, formerly of Reading, half-volleyed just over the bar.
After a bright start Chelsea were unsettled and it too considerable time before we were able to construct a meaningful attack once more. When we did, Azpilicueta's cross was cut out just before Torres could nod home.
Seven minutes before the break Mata came close to levelling, bringing down Ramires's long ball and turning past his man before shooting for goal, Vaclik just diverting it over with an instinctive save.
Just before the break, Torres headed over again from a Mikel cross. At least chances were still coming.
Neither side made changes at the interval, with Sparta set to counter, but it was Chelsea who almost scored on the break just a minute into the second half.
Oscar strode through midfield past a pair of challenges before finding the overlapping Ramires in the channel. The Brazilian shot first time, but saw his effort bounce back into play via a deflection and the outside of the post.
Not long later Torres went through but Vaclik saved well from the striker, and then palmed away a Terry flick that looked all the way like an equaliser.
The Blues continued to huff and puff, and Benitez called for Hazard and he considered changes. Oscar was the man sacrificed.
Moses, whose deflected effort was going wide, brought a diving save from Vaclik, as we approached the final 20 minutes. Pressure on Sparta's goal was beginning to increase, the visitors frequently operating with 11 men behind the ball.
A breakaway almost brought a second Sparta goal, but Cahill blocked from Kadlec Ramires dragged wide for Chelsea but then Tomas Zapotocny and Kadlec again both went close for the Czechs.
A Hazard free-kick, whipped from the left, evaded everybody but Vaclik, who punched clear despite an awkward bounce in front of him, and with the seconds ticking away Ramires's shot was blocked by a defender. Chelsea corner.
Mata crossed it low, Terry got something on it and it fell for Moses, on the swivel, six yards out. Somehow, the winger scooped it high.
As two minutes added time were announced it looked like we would be made to pay as Sparta broke again. Fortunately Kadlec shot wide. Then they broke again, and Cahill produced a stunning block on the same player.
Surely now extra time would come.
But wait. Hazard received possession down the left, and beat his man, cutting inside and back onto his right foot, before smashing an absolute rocket into the top corner of the Sparta net.
It was desperately cruel on Prague, but the Blues will not mind one bit. After the goal there was only time for sub Roman Bednar to be introduced before the referee blew the final whistle.
It is on to the last 16.
Chelsea (4-2-3-1): Cech; Azpilicueta, Cahill, Terry (c), Bertrand; Ramires, Mikel; Moses, Mata, Oscar (Hazard 67); Torres.
Unused substitutes Turnbull, Cole, David Luiz, Ferreira, Marin, Benayoun.
Goal Hazard 90+1
Booked Bertrand 85
Sparta Prague (4-2-3-1): Vaclik; Zapotocny, Svejdik, Holek, Hybs; Matejovsky (c), Vacha; Prikryl, Kadlec (Bednar 90+3), Krejci; Lafata (Kweuke 82).
Unused substitutues M Cech, Pamic, Vidlicka, Polom, Janos.
Goal Lafata 16
Booked Lafata 43, Hybs 55, Prikryl 84, Matejovsky 87, Kwueke 90+1
Referee Aleksandar Stavrev (Macedonia)