Eden Hazard's equalizer made it seem like Chelsea would escape Vicarage Road with a draw, but Watford stung late and often in a shocking rout.
Watford put in a dominant display from start to finish as they netted four times against Chelsea's Belgium keeper. The Blues looked like they may have salvaged a draw when Eden Hazard equalized in the 82nd minute. However, the home side hit back instantly with three goals in quick succession to earn themselves a deserved victory.
Watford started the game brightly, creating multiple chances in the first ten minutes. Gerard Deulofeu and Richarlison were both causing Chelsea problems and it looked like the Hornets would go ahead when Troy Deeney was left free at the far post for a Watford corner, but the striker miss hit the ball and it flew wide.
Chelsea had their first chance in the 12th minute when Willian shot over. But it was the home side that continued to look most likely to get the opener at Vicarage Road.
Just before the half-hour mark Chelsea found themselves down to ten men as Tiemoue Bakayoko was shown a second yellow for a foul on Richarlison.
In the 41st minute, things got worse for Chelsea. Blues' keeper Thibaut Courtois brought down Deulofeu in the box and Deeney made no mistake this time, converting calmly from the penalty spot, to give Watford a deserved lead.
Watford continued to be a constant threat for the Chelsea defense, powering forward at every opportunity. Chelsea introduced Olivier Giroud in the 64th minute as they continued to search for an equalizer.
Watford had another great chance in the 66th minute when Abdoulaye Doucoure surged through the Chelsea defense before taking his shot at goal, yet Courtois made the save.
The game opened up a bit with 20 minutes to go and the visitors put together a few attacks but struggled to carve out any clear-cut chances. Although Watford could not find that second goal they looked in control for most parts of the game until the final 10 minutes.
Cesc Fabregas thought he had the equalizer in the 81st minute, but the Watford keeper did well to get down low to his left and make the save, only for Eden Hazard to level things up with a long shot seconds later.
Watford, determined to restore their lead, continued to press the Chelsea defense and Daryl Janmaat put the home side back in front just two minutes after Chelsea thought they had earned themselves a point.
Deulofeu finally got the goal he deserved when he broke with the ball from inside his own half, ran through the Chelsea defense and slotted his shot past the Blues keeper.
Substitute Roberto Pereya rounded off a spectacular Watford performance as he made it four for the Hornets in stoppage time with a brilliant finish.
The result ramps up the pressure on manager Antonio Conte, with Chelsea following up its 3-0 home loss to Bournemouth with a dismal display and sitting just one point clear of fifth-place Tottenham.