Watford scored three late goals to win Javi Gracia’s first home game in charge and increase the pressure on Chelsea counterpart Antonio Conte.
The Blues played an hour with 10 men after Tiemoue Bakayoko’s red card capped off a dreadful individual 30-minute performance – in which he gifted Watford four chances.
The Hornets – who had only won one of their previous 12 Premier League games – led controversially as Gerard Deulofeu went down easily under keeper Thibaut Courtois’ challenge and Troy Deeney scored the penalty.
Chelsea, who brought on Olivier Giroud for a debut in the second half, scored an undeserved equaliser when Eden Hazard curled home brilliantly from 25 yards.
But two minutes later, Daryl Janmaat scored a fantastic goal of his own when he cut in from the right wing, played a one-two with Roberto Pereyra and beat another couple of defenders before scoring with his weaker left foot.
Deulofeu added another when he ran from halfway before his shot was deflected home by Gary Cahill – and then Pereyra scored a fourth from Abdoulaye Doucoure’s through ball.
It was Chelsea’s second consecutive defeat by three goals in the Premier League, following their 3-0 home reverse by Bournemouth.
Watford had earned just six points from 12 games since late November, with Marco Silva losing his job and being replaced by Gracia – who lost to Southampton in the FA Cup and drew 0-0 at Stoke in his opening two games, both away from home.
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