Liverpool remain near-certainties to win the title but they will not do it unbeaten. Some 423 days after their last Premier League defeat they suffered another and, if that in itself is difficult to comprehend given the relentlessness of Jurgen Klopp's men this season, then the manner of their downfall at Vicarage Road only adds to the sense of shock and awe.
For they were well beaten by a Watford side that came into this game in the bottom three and having failed to win any of their last six games.
But a performance of great togetherness and potency saw Nigel Person's men record a victory that will be talked about in these parts for years. Key to it was a sensational performance by Ismaila Sarr, who scored twice, set up the other for Troy Deeney and generally ran Liverpool ragged.
Klopp made two changes from the team that beat West Ham on Monday, with Dejan Lovren and Alex Oxlade-Chamberlain in for Joe Gomez and Naby Keita, absent with a minor fitness issue and a sore hip respectively.
Dejan Lovren's inclusion over Joel Matip was a surprise. Perhaps Klopp felt the Croatian would be better suited to dealing with the physical challenge presented by Deeney, who was yet again the lone forward in a Watford side that also contained two changes, the most notable of which was Sarr's inclusion over Roberto Pereyra. The Senegal winger was making his first start in over a month having recovered from a hamstring injury.
It was, however, Watford's other wide man who initially caused Liverpool problems. Twice inside the opening 10 minutes Gerard Deulofeu had attempts on goal after coming in from the left flank, the second of which was an effort that curled over the bar after he had easily got inside Trent Alexander-Arnold.
The Belgian was one of several players in yellow and black who caught the eye as Watford began brightly. They were well organised, tough in the tackle and, when they had the opportunity, broke forward in numbers, with Abdoulaye Doucouré another to have a shot towards goal before the interval.
Liverpool appeared vulnerable and were hardly helping themselves with their lack of cohesion and sloppiness in possession. Passes were overhit or misplaced and for the second time in a week they were making heavy weather of dealing with a team at the opposite end of the table.
Watford were well and truly in this game so it came as a notable blow for them when, on 37 minutes, Deulofeu had to be carried off on a stretcher having twisted his knee while battling for possession with Virgil van Dijk. The Belgian, so impressive up to that point, was down on the turf for around four minutes receiving treatment and the fear for the hosts will be that he is out of action for some time. He was replaced by Pereyra. Despite Deulofeu's withdrawal, it was the hosts who ended the half the strongest and indeed they produced their best chance of the game in added time - Deeney's curling effort from close range that fell just wide of the far post after the striker's initial attempt had been well saved by Alisson.
The opportunity arose from Liverpool's failure to properly clear a Watford free-kick, yet again highlighting their sloppiness and vulnerability during a first half in which they also failed to produce a shot on target.
The league leaders needed to wake up, yet they were again caught napping moments after the interval, allowing Sarr to collect Pereyra's through-pass and run clear on goal. The 22-year-old should have tested Alisson but instead hit his resulting shot well over.
The player did hit the target on 54 minutes, however, and it was a moment that sent the majority of those in attendance into raptures. A goal for the hosts and a moment that Lovren in particular will want to forget in a hurry as he inexplicably allowed Adam Mesina's throw-in to bounce over his head and run into the path of Doucouré.
He in turn played the ball across the area and there was Sarr to turn the ball in for his fifth goal of the season.
Liverpool were rocking and six minutes later they were floored as Sarr struck again, sprinting onto Deeney's perfectly-weighted through pass before lifting the ball over the outrushing Alisson as he was once again one-on-one with the Brazilian.
Klopp reacted by bringing on Adam Lallana and Divock Origi for Georginio Wijnaldum and Oxlade-Chamberlain respectively.
Yet it did not prove a turning point as Watford continued to push forward and extended their lead further, exploiting yet more sloppiness by Liverpool as Sarr intercepted Alexander-Arnold's weak back pass and, under pressure from Alisson, played the ball to Deeney who curled it into an empty net. Game over. Stunning shock completed.
"One-nil is not decisive if you react well, but then we conceded a second one," Klopp said. "It's still not decisive, but then a third one, which we helped and had a big hand in."
The German was magnanimous in defeat, adding: "It was exactly the game, a lot of moments which Watford wanted. Congratulations. Really well done and well deserved. We have to accept it."
Sunday Indo Sport