The game, both managers were to admit afterwards, could have gone either way with Fulham hitting the woodwork on three occasions but Watford creating more chances.
Watford, who have already dispatched Premiership sides Southampton and Portsmouth in the Carling Cup on this ground earlier in the season, could have been excused if they had one eye on next week's semi-final first leg clash at Liverpool.
The struggling Championship side approached the game with commitment and a positive attitude and were rewarded when they cancelled out Fulham's 17th minute goal just before half time.
Fulham took the lead when Mark Pembridge floated over a free-kick towards the far post. Veteran keeper Alec Chamberlain went to cut out the ball but then had second thoughts and was caught flat-footed when Zat Knight headed firmly into the net.
Watford had survived an early scare when Collins John hit the far post with a wicked cross-shot but Watford came back strongly and should have taken the lead just before Fulham scored.
Heidar Helguson chested down a free-kick and turned to stab the ball just wide of the post with Edwin Van der Sar struggling.
Watford finally equalised in the 42nd minute. Helguson headed towards goal and captain Neil Cox seemed set to force the ball home only for Andy Cole to clear off the line.
As Hameur Bouazza moved on to the loose ball, Liam Fontaine pulled him back and the referee pointed to the spot. Helguson coolly planted the spot-kick into then net.
Cole hit the post early in the second half and Steed Malbranque fired against the bar. For Watford, Helguson headed over the bar with the goal at his mercy and the Fulham keeper saved shots from Bouazza, Anthony McNamee and Gavin Mahon.