West Ham's record signing Sebastien Haller scored a brace to secure a 3-1 win and condemn Watford to a third-successive Premier League defeat this season.
Captain Mark Noble converted from the penalty spot inside the opening minutes before Andre Gray hauled the hosts level with 17 minutes gone.
But Watford were made to pay for spurning a number of chances as Haller, the £45million summer arrival from Frankfurt, scored twice in nine second-half minutes to heap the pressure on manager Javi Gracia, whose side have now lost their last seven matches in all competitions.
Despite missing the opening two games through injury, Noble was thrown back into Manuel Pellegrini's starting line-up and – on the 15th anniversary of his debut – the West Ham skipper put his side ahead.
With just 59 seconds on the clock at Vicarage Road, Manuel Lanzini fell in the box following a clumsy challenge from Abdoulaye Doucoure. Referee Chris Kavanagh did not hesitate in pointing to the spot.
In his 468th West Ham appearance, Noble then made no mistake, sending goalkeeper Ben Foster the wrong way, to score the 25th penalty of his career.
Only Alan Shearer, Frank Lampard and Steven Gerrard have scored more times from the spot in the Premier League than the 32-year-old midfielder.
It was just the start Gracia would have been dreading, with Watford heading into Saturday's sun-bathed fixture without a victory since winning at Huddersfield on April 20.
But, to their credit, the Hornets were soon on the front foot, and probably should have been on level terms after six minutes, only for Gerard Deulofeu to graze the crossbar from a tight angle.
Moments later, the former Barcelona and Everton midfielder was through on goal but dithered, allowing Hammers centre-back Issa Diop to divert his shot for a corner following a fine sliding tackle.
The home side were on top, and they got their just rewards when Will Hughes' cute through ball was thrashed home by Gray, deputising for the injured Troy Deeney.
The game was wide open, with chances for both sides – the opening period concluding as Felipe Anderson's header from a corner struck the Watford woodwork.
The second half began as the first ended, with some fine attacking play, and some rather questionable defending.
Deulofeu, whose influence on the match was growing, played in Gray. His cross fell to Hughes at the far post, and the midfielder looked certain to double the hosts' tally for the campaign, yet he fired inexplicably wide.
At the other end, Michail Antonio – fresh from coming off the bench – was through on goal, only to be denied by an on-rushing Foster, who then leapt to his feet to disposes Anderson and clear the ball to safety.
The former England international, however, could do nothing about Haller's ensuing strike.
Anderson popped up on the left-hand side and his perfect cut back found Haller, who could not miss from six yards.
West Ham's tails were up, and the hosts had Foster to thank for a fine point-blank save from Antonio's header.
From the resulting corner, Foster was called into action again, tipping Antonio's header on to the bar.
But to the Watford stopper's dismay, the ball dropped off the woodwork to Haller, whose expert bicycle kick sealed all three points for Pellegrini's men to leave Gracia facing further questions over his Watford future.
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