As hundreds of fans and I walked through the rain from Seven Sisters station to the Tottenham Hotspur Stadium, there was a real sense of tension in the air, and with good reason.
This was the night that would not only define Spurs' season but also most likely the wider future of the club. If they were to end it with their hopes of finally winning a trophy and qualifying for European football next season over, it was difficult to see how Postecoglou would last beyond the summer.
That was especially the case because the injury crisis that he's been able to blame his side's struggles on was largely over. Dejan Kulusevski missed out and Rodrigo Bentancur was suspended, but the rest of his key players were fit and in the starting XI.
"The time for talking is over - we need to deliver on the pitch," said the manager in the pre-match programme, and he was right. No more excuses, it was now or never.
As a full moon rose above the ground, the tension gave way to a roaring atmosphere sparked by a rousing rendition of Oh When The Spurs Go Marching In, and the home side responded.
They controlled the game from the off and took the lead through Wilson Odobert after 26 minutes, drawing level with AZ on aggregate.
They didn't blow their opponents away after that but remained in complete control and took the lead of the tie at the start of the second half courtesy of a cool finish from James Maddison.
AZ then took advantage of a defensive mishap to peg them back but Spurs regrouped, regained the upper hand through Odobert and never really looked like relinquishing it.
Read a full report of Tottenham's win here.
Postecoglou would've no doubt breathed a huge sigh of relief at full-time, and it was not only the result the Aussie desperately needed but a convincing performance that would've restored Daniel Levy's perhaps wavering faith in him too.
With a squad finally close to full strength, there was real quality in every area of the pitch.
The story of the night was Odobert, who scored his first goals since returning from an injury he picked up all the way back in September.
Both were chances he was expected to score, but he showed intelligence to be in the right place at the right time and a lot of composure in front of goal.
He did more than just find the back of the net too, causing AZ a lot of problems with his direct running and working hard off the ball.
"I'd be surprised if I have to explain what I saw in him, if you didn't see that today," Postecoglou told me in the post-match press conference.
"Super pleased for him to get his goal tonight. He's worked hard silently to get back in the team.
"Obviously he came to a new club this year, a big move for him, and it just hasn't worked out through no fault of his own because of his injury.
"It's been frustrating for me, not being able to showcase him knowing how much he can develop, and this is just a starting point for him, he's still working his way to full fitness."
Odobert and Lucas Bergvall were at fault for the AZ goal, which is ironic given the Swede was another of Spurs' standout players.
His potential has been clear all season but with a solid defensive platform behind him provided by the returns of Cristian Romero and Micky van de Ven, the 19-year-old thrived, dictating the game at the heart of the midfield.
He found a teammate with nearly every pass (44/48), jinked past opponents and wooed the crowd with some lovely moves under pressure. Off the ball, he constantly won back possession in what was a complete performance at the heart of the midfield.
If AZ did get through him and his fellow midfielders, Van de Ven was there to single-handedly clean things up with his otherworldly physical attributes.
When the visitors got in behind Spurs, the Dutchman would outpace his opponent with ease and had the composure to carry the ball out of defence after winning it, getting his side back on the front foot.
It's not a coincidence that Spurs conceded on their left almost immediately after the centre-back was taken off, and Postecoglou admitted that they struggled without him.
The manager would probably also admit that they're too reliant on him, but he's a defensive weapon that precious few sides in football have when he's fit.
With him and Odobert back in the team and adding a huge amount, Bergvall and a number of other players - Son Heung-min produced his best performance in a long time - flourished and this suddenly looked like a world-class side again.
It may not have been as famous a European night against a Dutch side as when they beat Ajax in the Champions League semi-finals, but if it revives Postecoglou's project and marks the start of a run to their first European trophy since 1984, it could ultimately prove to be just as important.
