Odion Ighalo was on target once again as Watford made it back-to-back wins and ended West Ham's unbeaten away Premier League record with a 2-0 triumph at Vicarage Road.
West Ham entered Saturday's contest riding high in third place having beaten Chelsea 2-1 at Upton Park to add to notable away wins at Manchester City, Liverpool and Arsenal already this term.
However, Slaven Bilic's men failed to hit those same standards and a first-half own goal from Aaron Cresswell and Ighalo's sixth of the season helped Watford to an impressive win.
Andy Carroll was West Ham's hero in scoring the winner against Chelsea last weekend, but the England striker's indecisiveness when making a routine clearance led to Watford's opener in the 39th minute.
It was a deserved lead for Watford and the home fans were celebrating again early in the second half thanks to Ighalo's emphatic finish.
James Collins was dismissed towards the end to cap a miserable outing for the Hammers, who have taken just one point from games against the three promoted sides in the league this term.
A dominant Watford side were well worth the full three points, however, and could have led as early as the eighth minute.
Nathan Ake's header from Ben Watson's corner drew a fine point-blank save from Adrian and referee Keith Stroud waved away penalty claims after Carroll nodded Troy Deeney's follow up against his own arm.
The hosts had another claim for a penalty when Almen Abdi's flick appeared to hit Collins' hand, Ighalo played on and forced Adrian into another smart stop.
But the pressure finally told. Carroll - making his first Premier League start since February - dawdled in the area and Ake stole in before squaring across goal for Ighalo, who pressured Cresswell to turn into his own net.
Another West Ham error helped the Hornets three minutes after the interval. James Tomkins made a hash of Ikechi Anya's low cross from the left allowing Ighalo to take a touch out of his feet before hammering his shot into the left-hand corner.
Adrian was equal to Ighalo one-on-one as Watford turned the screw and Deeney miscued badly from the edge of the box after Etienne Capoue did well in midfield.
Carroll sliced a volley wide and Enner Valencia hit the post as Bilic's men tried to mount a late comeback.
But their momentum was checked as Collins was handed his marching for orders for going through the back of Ighalo in the 84th minute, helping Watford cruise to a first home win over West Ham in the league since 1985.
