WATFORD recorded their first away, and first back-to-back win of the campaign as Anthony McNamee's overhead kick condemned West Ham United to a fourth round exit at the Boleyn Ground.
His goal, perfectly timed before half-time, secured the narrowest of victories for Aidy Boothroyd's troops who return to Upton Park in two weeks time for a Premier League fixture.
And it was little more than the visitors deserved after a 90 minute display of pride and passion left the home fans jeering their own side's torrid efforts.
Just 44 miles separates the two sides, making the fourth round tie between them the third closest in proximity, but yet again the millions between the two club's budgets were self-evident on the team sheets.
Hornets' boss Aidy Boothroyd welcomed new signing Steve Kabba into his squad, but found himself without the services of skipper Gavin Mahon, Dan Shittu and the cup-tied pairing of Will Hoskins and Lee Williamson.
Alan Curbishley meanwhile, despite his defensive problems, had the luxury of drafting in the likes of new signing Lucas Neill, Bobby Zamora and Jonathan Spector.
West Ham, the last FA Cup winners from outside the top division, started quickly on their home turf and within two minutes had Ben Foster's crossbar rattling dangerously.
Seconds earlier Foster had been adjudged to have nudged the ball out for a corner kick as referee Howard Webb allowed his assistant to overrule his initial decision of awarding a goal kick.
And from Luis Boa Morte's set play Zamora leapt at the back post to flight a looping header off the visitor's metalwork.
Despite that early scare Watford soon settled and began to share the majority of the play, almost forcing an opener of their own on the quarter-hour mark.
Prolonged Hornets' pressure resulted in McNamee finding space to float a long diagonal cross to former Hammer Malky Mackay.
Unselfishly the Scot headed back across goal to the waiting Damien Francis who touched home into the net before seeing an offside flag raised on the far side.
This scare seemed to spark the home side into life and they soon began to wrestle control of the tie back midway through the half.
With Watford's defence dropping deep into their own half Boa Morte drove through the centre of midfield to find Shaun Newton, whose low shot cannoned off Jordan Stewart and then the body of Foster.
The Hammers kept the pressure on and seconds later Stewart was again on hand to clear off the line from the head of Christian Dailly.
In what was a scrappy first half affair both sides continued to create chances and soon after it was Watford's turn to force the opposition into evasive goal line action.
Following a Hornets' right-wing corner Darius Henderson was on hand to head back across goal, where West Ham's Carlton Cole did well to reach the ball and put it over his own bar under severe pressure from the leaping Francis.
Getting back to what he knows best Cole was quickly on the mark at the other end of the field this time sending a rasping low shot just inches wide of Foster.
There was little doubt that Watford were causing United's makeshift defence problems and so it proved when the Hornets broke into the lead with just three minutes remaining in the half.
A routine Hameur Bouazza cross, which should have been easily dealt with by Roy Carroll, was missed by the former Manchester United 'keeper leaving McNamee the opportunity to send home a spectacular looking overhead kick unchallenged from 12-yards.
It was the prefect end to the half for the Hornets as McNamee's third senior goal sent their hefty following into the interval happy.
The second period started purposefully for the home side but they were forced into an early change as John Pantsil replaced the injured Neill just moments into the resumption.
It didn't serve to disrupt them too much though as Zamora demonstrated when he escaped the attentions of Mackay to drive straight at Foster 50 minutes in.
Zamora was again on hand to head narrowly over from their next attempt, after Newton had flashed over a right-wing cross.
On the hour mark West Ham had penalty appeals waved away as Cole went down in the area under pressure from Mackay, resulting in a yellow card for diving being shown in the direction of the United forward.
Looking for a way back into this one-off tie Curbishley threw on the legendary 40-year-old forward Teddy Sheringham, in place of Newton.
But even that failed to weld the Irons together as Watford held strong with a passionate and uncompromising away day display.
Watford boss Boothroyd restructured his side with 18 minutes to play, moving another former Hammer Chris Powell into midfield, in place of scorer McNamee.
Soon after Curbishley made his third and final change with the substitution of Matthew Etherington for the ineffective Boa Morte.
After Francis had been booked late on for a foul on Nigel Quashie, Spector gave the visitors one final scare, forcing the ball goal-wards where stand-in skipper Stewart would clear.
But still the Yellow Army held strong, galvanised as a unit through to the final whistle, which signalled wild scenes amongst fans and players alike.
The last time these two sides met in this competition was in the previous round in 1994, with the East Londoners coming out on top that afternoon 2-1.
Today though was a day for the Hornets, who now go into Monday afternoon's fifth round draw.
WEST HAM UNITED (4-4-2): Carroll; Neill (Pantsil 49), Dailly, Spector, McCarthy; Newton (Sheringham 65), Reo-Coker (c), Quashie, Boa Morte (Etherington 77); Cole, Zamora. Subs not used: Green (gk), Mullins.
WATFORD (4-4-2): Foster; Mariappa, Mackay, DeMerit, Stewart (c); Smith, Bangura, Francis, McNamee (Powell 72); Henderson, Bouazza. Subs not used: Chamberlain (gk), Ashikodi, Kabba, Doyley.
BOOKINGS: WEST HAM UNITED; Neill (44 - foul), Cole (59 - simulation). WATFORD; Francis (85 - foul).
GOALS: WEST HAM UNITED; none. WATFORD; McNamee (42).
REFEREE: Howard Webb
ATTENDANCE: 31,168
