Sergio Aguero scored a hat-trick as Manchester City moved top of the table and handed Watford their heaviest ever top-flight defeat.
Previously unbeaten Watford could have also topped the Premier League with a win against Pep Guardiola's side on Saturday, but they were undone by City's prolific attack who have now scored 15 goals in three matches.
Aguero headed home a Kevin de Bruyne free-kick before tapping in from close range four minutes later, after brilliant build-up play from Gabriel Jesus and a final pass from David Silva.
Jesus added a third before half-time - his fifth of the season - when he drilled home from Aguero's cut-back, despite tame calls for offside from the Hornets defence.
Watford had kept three clean sheets this season, and came out reinvigorated after half-time, with Andre Carrillo and Richarlison both having decent chances.
But City continued to push forward and added three more.
Nicolas Otamendi headed in David Silva's corner before Aguero completed his hat-trick when he poked home after a pulsating solo run into the box.
Raheem Sterling completed the rout from the penalty spot, after he was brought down by Christian Kabasele.