Walter Mazzarri heralded his first victory as Watford manager as the beginning of "great things" after West Ham were over-run 4-2 at London Stadium.
The Hornets trailed 2-0 shortly after the half hour mark due to a brace from Michail Antonio, but by the interval they were level through strikes from Odion Ighalo and Troy Deeney.
West Ham's defence continued to crumble and Etienne Capoue and Jose Holebas capitalised to give Mazzarri a first Premier League win in his four matches since taking charge at Vicarage Road.
"I'm very, very happy, particularly because we won a game that we were losing 2-0 against such a big team and in such a big stadium," Mazzarri said.
"For sure this gives a lot of confidence to the players and the whole team. I'm a new manager here at Watford, but slowly we are going to build great things."
West Ham seemed destined for a comfortable win until Ighalo's 41st-minute strike launched Watford's fightback, but Mazzarri never gave up hope of producing an unlikely upset.
"I always tell my players to not think so much about what has happened, but to think about what they can do. I'm very happy my players have this mentality," he said.
"Another concept I tell my players is that they must believe they can win, always, until the referee blows the whistle for the end of the game.
"We were very afraid early on, especially in defence, and this was because West Ham played very well in the beginning.
"They were pressuring us, but we felt not only the pressure of them but also the public.
"After that we had a couple of opportunities where we could have scored, but we didn't.
"At half time we spoke with the players, we adjusted a couple of things and then the result came our way.
"We did very well in the second half and it's because of us that we had the win and not because of the poor defence of West Ham."
Slaven Bilic refused to excuse the defensive implosion that saw West Ham's two-goal lead crumble to a 4-2 defeat.
"It's hard to explain. It was way too easy. If we defend like this we aren't going to win too many games because we scored enough and created enough," Bilic said.
"I am very angry. I am, I am. At half-time some of the players were already pale, so I didn't want to give them more criticism.
"Watford's second was a classic for YouTube because there was no communication between two players. That happens. Of course it shouldn't, but you don't expect it to happen again.
"It's more the first goal that worries me because it came from a very predictable attack. They should have to do something better to hurt us."
Source: PA
