Watford recorded their first victory of the season after coming from two goals behind at the London Stadium to run out 4-2 winners.
Michail Antonio and Dimitri Payet combined to give the Hammers a comfortable lead, only for Walter Mazzarri’s men to spring into action on an afternoon of high drama at London Stadium.
Odion Ighalo, Troy Deeney, Etienne Capoue and Jose Holebas were on target as the home defence fell apart and, but for a little more accuracy in the final 10 minutes, Watford could have won by more.
The first half alone produced four goals, Antonio plundering two of them as West Ham seized a 2-0 lead that owed much to the vision of France midfielder Payet.
Payet, starting his first match of the season after recovering from a thigh problem, provided the corner for Antonio’s first before delivering a stunning rabona cross for his second.
Almost as captivating was Deeney’s lob of goalkeeper Adrian in first-half injury-time that enabled Watford to enter half-time all-square and West Ham’s fretful back four never recovered.
A 56,974 crowd at London Stadium was brought to its feet right from the start as West Ham grabbed a fifth-minute lead when Antonio headed home a corner by Payet.
Antonio and Payet had already forced smart early saves from Watford keeper Heurelho Gomes, who then watched in horror as an inexplicable back pass from short range by Daryl Janmaat struck the left post.
Chances were also falling at the other end, however, with Ighalo at fault for missing one before Adrian came to West Ham’s rescue for the second.
The high-octane start gave way to a more measured spell, but the home team remained in control with Antonio offering their best avenue of attack down the right.
It was in the same area of the pitch that Payet produced a moment of magic that enabled West Ham to extend their lead, using his footwork to torment Watford’s defence before executing a rabona cross to the far post.
Antonio needed to time is run correctly and he did exactly that, ghosting clear of his marker to nod home.
The thrills continued when Watford reduced the deficit five minutes before the interval with Ighalo turning Sam Bryan inside-out before stabbing home a shot that deflected off James Collins.
And there was still time in a dramatic first half for the visitors to snatch an equaliser after a dreadful mix-up between Collins and Adrian left the Hammers keeper in no man’s land.
Deeney still faced the trickiest of chances, but the skipper was inch perfect as he lobbed the ball over Adrian and into the bottom right hand corner.
West Ham’s crumbling defence then allowed a cross by full debutant Roberto Pereyra to sail over their heads to an unmarked Capoue, who rifled the ball home from close range to give Watford the lead for the first time.
Once more the home defence was exposed as Holebas drilled a pass by Ighalo into the back of the net, but they also cried foul after seeing Payet brought down in the build-up.
The complaints fell on deaf ears and the defenders could only have themselves to blame for allowing Watford to post an unlikely victory.