English football has never seen a team as good as Manchester City were in 2017/18.
They may not have had the long-term sustainability of Sir Alex Ferguson's Manchester United or the dogged invincibility of Arsene Wenger's.well, Invincibles, but as far as performances within a single season go, Pep Guardiola's Centurions were the best of the best.
When Liverpool inflicted their first defeat of the season in January, then, it was groundbreaking, but it was generally viewed as a bit of a fluke.
In one of the wildest games the Premier League has ever seen, the Reds charged into a 4-1 lead that had City's heads spinning. It looked as if Jurgen Klopp's high-pressing system was the antidote to Guardiola's possession-based dominance, and there were whispers emerging that Liverpool had the secret weapon to end City's reign of terror.
Yet the hectic nature of the game and the fragility Liverpool exhibited in the eventual 4-3 win suggested they had a long way to go before they could even dream of rivalling the dominant force from Manchester. It was a one-off, and that's what City were out to prove when the two sides were drawn together in the Champions League quarter-finals.
Within half an hour of their return to a raucous Anfield, though, Liverpool made it clear to the Premier League leaders that they had something to be very worried about. City didn't know where to look; just as they had been months earlier, they were being hounded at every turn.
Every time Liverpool regained possession (that happened a lot), they looked like scoring. It was 11 vs 11, but Guardiola's team looked comprehensively outnumbered.
City's hapless defensive performance was summarised within ten minutes when their entire back-line was dragged over to Roberto Firmino, only for the Brazilian to poke the ball into the path of Mohamed Salah. Ederson was left scrambling, Salah did the necessary, and Anfield went off.
The visitors had tried to throw Liverpool off their stride by making them attack the Kop end in the first half, but that backfired spectacularly. The deafening noise behind the City defence evidently unsettled them, and it only got louder when Alex Oxlade-Chamberlain delivered his finest moment in a Liverpool shirt.
It came after James Milner flew into a challenge to nick the ball from Ilkay Gundogan. Oxlade-Chamberlain, controlling the loose ball, was only going to do one thing from there. One touch, and then it exploded beyond Ederson.
Cut to Pep Guardiola, shrugging his shoulders on the touchline with no idea what to do for the best.
When Sadio Mane headed in Salah's cross on the half hour mark, it genuinely seemed like it could have been double figures. City didn't know how to defend against Liverpool, and it would take them a while to learn.
It would be more than two years before Klopp's team finally broke City's stranglehold over English football, but this was an early sign that the tides were changing, and the Premier League champions-elect had weakness after all.
It was a spectacular European night at Anfield, and it set the wheels in motion for an era-defining rivalry.