Manchester City – Premier League champion, elect in the eyes of many – now know they are in a real scrap after Liverpool leapfrogged them to the top of the table on a telling day in the title race.
Both sides started Saturday unbeaten after 15 matches with an impressive 25 wins between them – but it was Pep Guardiola's champions who blinked first.
Mohamed Salah scored a hat-trick for Jurgen Klopp's side as they romped to a 4-0 win at Bournemouth in the early kick-off, taking them to 42 points, one clear of City.
City had the tougher task of taking on Chelsea at Stamford Bridge later in the day and despite dominating the first half they ended up losing 2-0 – their first league defeat since April.
City remain firm favourites to win their second consecutive title but their aura of invincibility has been shaken and a hungry Liverpool are not in the mood to back off.
Guardiola took pride in his team's performance despite the defeat, saying he never felt they were going to match the Arsenal Invincibles, who went through an entire Premier League season unbeaten in 2003-04.
"We are going to try to be champions," he said. "In a season like this, with a lot of games – plus after last season – everybody wants to beat us. They put in something extra.
"What is important is how we react. Even if we would have won today, it's still only the beginning of December. There are still a lot of points to play for."
For all of City's much-vaunted squad depth, they sorely missed the injured Sergio Aguero at Stamford Bridge, mustering 14 shots but managing just four on target.
Kevin De Bruyne - last season's club player of the year – has barely featured during this campaign because of injury and although City have an enviable supply of creative players, it is difficult to replace a player of that standard.
Liverpool have not been playing with the same fluency as they did last season but they have added steel to their game and boast a phenomenal defensive record, with just six goals conceded in 16 Premier League matches.
The addition of centre-back Virgil van Dijk and goalkeeper Alisson Becker has made an enormous difference at the back and at the other end of the pitch, Salah has started firing again.
The Egyptian forward had a quiet start to the campaign by the standards of last season, in which he scored 44 goals in total, but is now the Premier League's joint-leading scorer, with 10 goals.
The manner in which he took his third goal yesterday, in particular, showed a player who is back to his best after a nasty shoulder injury in the Champions League final appeared to knock his confidence.
Liverpool are also showing the resilience of would-be champions – scoring deep into injury time last week to beat Everton and battling back from a goal behind against Burnley in midweek.
It is almost impossible to overestimate what winning the Premier League would mean to Liverpool fans.
Last crowned champions of England in 1990 – before the birth of the Premier League – they had to stand by and watch as Manchester United overhauled their proud record of 18 top-flight titles.
Klopp said: "The thing is that we just really want to be as successful as possible. Nobody knows what that means, nobody.
"This league is so difficult to play in, but 42 points I usually had in April and now it is December. That's massive and it's exceptional and it is all to the boys, they deserve all the credit for that because they really fight for each yard."
Guardiola does not think it is going to be just a two-horse race.
Arsenal, under new manager Unai Emery, are on a 21-game unbeaten run in all competitions and Tottenham are only six points off top spot while Chelsea are still in the mix.
"We are only in December," said the City boss.
"Can Liverpool win the Premier League? Yes. Ask me if Tottenham, Arsenal or Chelsea can – yes, of course."