Manchester City conceded more than one goal in a Premier League game for the first time since April
Manchester City have blinked first, Liverpool have taken over at the top, and the Premier League title race is well and truly on.
Pep Guardiola's reigning champions will remain favourites because of sheer weight of quality, but Liverpool hovered in their shadow in second place before their 4-0 win at Bournemouth and City's defeat at Chelsea sent them to the front.
The Reds are now the only unbeaten team in the division so how, if at all, has this 90 minutes at Stamford Bridge changed the face of the title race?
Man City stumble lets in Liverpool
A second successive Premier League title for Guardiola and his side has been regarded as a formality in many quarters given the imperious nature of their performances and victories in the first 15 games of the season.
It should be stressed this was never the view of the manager or his players as they swept a succession of teams aside in the purist style that has become their trademark.
And what those propagating the premature coronation ignored was Liverpool lurking doggedly on City's shoulders, refusing to buckle while grinding out results rather than blitzing teams as they did last season.
City will not overreact to one loss, no matter how disappointing. But there is no question this slight shift in the landscape adds an intriguing dimension to the sharp end of the Premier League.
The Blues' first league defeat of the season is likely to have been digested then forgotten by the time they return to the training ground - especially as for the first 44 minutes they performed superbly without the crucial final flourish to reward their work.
Guardiola believes his team are operating at the same level as last season but opponents are gathering momentum, in this case particularly Liverpool.
He was predictably unmoved by the loss, pointing out the date was only December, talking up Chelsea, Arsenal and Spurs as title rivals as well as Liverpool, and insisting invincibility is irrelevant when set against City's ultimate goal.
It was others, not Guardiola, who applied the aura of invincibility that was stripped away here, and he said: "We are not here to be invincible, we are here to be champions. If people talk about people being unbeatable, you are selling an illusion."
City's failing here was a lack of ruthlessness and proof, no matter how powerful and multi-talented your squad, that eventually a time will come when big players are missed.
Guardiola refused to be drawn into the argument, praising those who did play, but City missed the injured Sergio Aguero and Kevin de Bruyne. No question.
It was certainly the case as they dominated the first half without reward. Big players influence big games and City's loss was two-fold - both when they could not profit from total dominance and then when they chased the game after N'Golo Kante's goal.
City have such class and quality they will recover. The smart money is still on them to win a title pursuit that has 22 games to go - but this loss was not so much a reality check for them as for those of us who were in the mood to present the crown now rather than when it is actually won.
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Liverpool can hardly be described as the tortoise compared to City's hare - but there is little doubt they have lived in their slipstream this season.
And now, as they lead the league on merit and the scenery shifts, their manager Jurgen Klopp can inspect some vital signs that will give him huge encouragement as he sets his sights on Liverpool's first title since 1990.
The Reds now stand alone as the league's only unbeaten team, but this is only one of the factors that will bolster their growing confidence.
The latest clean sheet at Bournemouth means they have conceded only six league goals this season as Klopp's side feel the benefits of the £75m signing of Virgil van Dijk in January and the £67m summer addition of goalkeeper Alisson.
Liverpool showed spirit and squad strength to come from a goal down to win at Burnley in midweek and, ominously for their rivals, the win over Bournemouth saw Mohamed Salah come closest to the devastating form that brought him 44 goals last season.
Salah's hat-trick had echoes of that campaign of destruction and if he is running into that form again, Liverpool will take a lot of stopping.
Their biggest supporters would struggle to make a case for them playing with the same blistering flamboyance as last season - but the other compelling side of that argument is they are now ahead of a team who have been beating all-comers without being at their best themselves.
They have every right to feel there is more to come.
It is a formidable achievement so far. Now Liverpool face the different pressure of leading from the front.
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It may look like it as City stand seven points clear of Chelsea in third - but Guardiola is not having it, and in December all remains possible.
Liverpool and City have been the two outstanding teams this season but Chelsea, indifferent recently, delivered a reminder of their credentials with this win, while Spurs and Arsenal were also name-checked by the City manager.
Crucial weekends lie ahead as Liverpool will hope for a favour from neighbours Everton when they go to City next weekend, and Guardiola will look for an unlikely ally in Jose Mourinho as he takes Manchester United to Anfield.
And one fixture is already ringed in red for its importance - and the possibility to strike a psychological blow.
That is when Liverpool travel to City on 3 January. Hardly winner takes all, but hugely important.
One thing is certain.
The result that sends Liverpool leapfrogging over City means the Premier League scenery will now be viewed in a different light.