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Slaven Bilic insisted West Ham's 3-1 Barclays Premier League win over Watford was crucial in their bid to reach the top four.
Andy Carroll hit his sixth goal in six games and Mark Noble tucked away two penalties to move the Hammers to within four points of fourth-placed Arsenal.
Bilic felt if West Ham failed to win they could kiss goodbye to their Champions League dream.
Instead, the Upton Park boss has targeted four victories from their last four matches to crank up the pressure on Arsenal, Manchester City and Manchester United.
"I told the players beforehand that this is kind of like a decisive game," he said. "This will determine if we are going to look up, or look down.
"It's nice to have a cushion below and it's nice to be three points behind United and to close the gap to City.
"We now have a break and we are going to use it to rest, then come back in really good shape for the last four games of the season. We're going to give everything to win them."
The Hornets clearly had one eye on Sunday's FA Cup semi-final against Crystal Palace with boss Quique Sanchez Flores making seven changes.
Yet they still got one back through Sebastian Prodl and had a late penalty of their own saved before Nordin Amrabat was sent off in injury time.
Dimitri Payet set up Carroll for the 11th-minute opener, with the striker guiding his cross past Heurelho Gomes with the outside of his left shin.
West Ham passed up chances to stretch the lead until r eferee Mike Dean pointed to the spot in the 45th minute after Jose Holebas felled Cheikhou Kouyate.
Gomes memorably saved two penalties from West Brom's Saido Berahino on Saturday, but was unable to record a hat-trick as Noble sent him the wrong way from the spot.
Almen Albi's foul on Michail Antonio gifted Noble his second and West Ham's third before Prodl volleyed home.
In stoppage time Dean awarded a third penalty of the night when Angelo Ogbonna grappled with Prodl, but Adrian saved substitute Troy Deeney's spot-kick.
Moments later Amrabat saw his hopes of a Wembley appearance ended by a second booking for a foul on Winston Reid.
"A second yellow was too much," moaned Hornets boss Sanchez. "It was normal, not enough to be a second yellow.
"We conceded two penalties again and the first goal we reacted too slowly to Carroll, so we have to analyse that.
"We planned to make a lot of changes but I think we were competitive and we created chances."
Sanchez is also dealing with speculation surrounding his position amid reports Watford's owners are not happy at their form since the turn of the year - they have won just three league matches in 2016.
The Spaniard added: "We have an amazing time now - we have 41 points, the position is very comfortable we are in semi-final of FA Cup. The future we will see what happens."