Bottle. Courage. Nerve. Call it what you like, but that's what it came down to in this extraordinary affair after 210 minutes failed to separate these sides.
Perhaps sheer force of will was also involved, since a rampant Anfield, you felt, just would not be denied on a night that compares with all the classics in Liverpool's proud history, if you use tension and stress as a yardstick.
And talking of bottle, you have to give a mention to the man who set the ball rolling, the man who needed a solid game for his own peace of mind.
In scoring the opener on this remarkable night, Daniel Agger had answered an awful lot of questions. In side-footing home a beauty from the edge of the penalty area, Liverpool's centre-half had shown a great deal of character and, in doing so, maybe put his own mind at rest.
Thinking about it now, it must have been a miserable six days for the 22-year-old. The events of Stamford Bridge had surely seen to that after the defender had been horribly exposed by the raging bull known as Didier Drogba. For Chelsea's powerful centre-forward, it was just one of those games when he came out on top in almost every single tussle.
For Agger, what's more, this was a fairly new experience. Since joining from Brondby last summer, this composed, ball-playing Dane had rarely come second best in one-on-ones. His debut Premiership season, in fact, was turning into an unqualified success.
You can imagine his angst, then, after being given the runaround in such a huge game. The memory was never going to be cleansed until the chance came to make amends.
It was obvious from the start last night that Drogba fully intended to try his luck again. Pulling across to Agger's side, the striker attempted to outmuscle his opponent once more, the difference being this time that Liverpool's No 5 appeared stronger - more prepared, maybe - than he had in west London.
Staying tight, giving as good as he got, Agger was determined to avoid another embarrassment. At one point, that seemed to involve crashing into his opponent from behind as the ball came fizzing in. Closer inspection revealed that a frustrated Drogba was just trying it on, holding his head theatrically when no contact had been made.
Helping Agger on his way, though, was Liverpool's added resilience as a team. Spurred on by deafening support from a throbbing Kop, a more compact outfit rarely left gaps for their marauding visitor. Jamie Carragher, for one, was seeing to that. As the absolute embodiment of Scouse defiance, Carragher kept on yelling across to Agger over the incessant noise, offering advice, making sure that his partner didn't switch off.
On the one occasion in the first half that Drogba did wriggle free, his shot pummelled into Jose Reina's body and away to safety.
That chance came just a few minutes after the only goal. With Steven Gerrard lining up a free-kick on the left, few of the Chelsea players expected a square ball. Even so, there was an awful lot of work to do from 17 yards out, but Agger swept the ball home beautifully with his left foot, his shot perfectly placed to beat Petr Cech's grasping dive.
With the crowd serenading Liverpool manager Rafael Benitez at the end, the Spaniard came across to hug his man.
Benitez is known as a manager who doesn't get close to his players, who doesn't give many pats on the back when performances deserve it. Well, if that is true, he was clearly prepared to make an exception here.
One of his key players had stayed firm and resolute to help set up a night in Athens that will surely require a bit more bottle still. In that particular direction, Agger has proved that he doesn't come up short.
Copyright - The Telegraph