Record signing Sebastien Haller scored a second-half brace as West Ham consigned Watford to a third consecutive Premier League defeat with a 3-1 win.
The French striker who arrived from Eintracht Frankfurt in a deal worth up to £45m this summer opened his West Ham account with a tap-in from Felipe Anderson’s cut-back.
And he then acrobatically turned the ball home with an overhead kick from three yards after following up from a corner.
Mark Noble’s third-minute penalty set West Ham on their way to their first win of the season after Manuel Lanzini had been fouled by Abdoulaye Doucoure.
But Watford responded to level with a well-worked Andre Gray goal, with the forward drilling in a right-foot shot from Will Hughes’ pass.
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However the Hornets were punished for their wastefulness in front of goal, as they registered just three of 23 shots on target.
Javi Gracia’s side, who sit bottom of the table, have now lost seven successive games in all competitions for the first time since 1999, while West Ham’s victory sees them climb to 13th.