Both teams went into the game having impressed in the final third this season, but the opening 45 minutes in Hertfordshire featured very few opportunities.
Watford hit back with Deeney powering a header of his own off target, but that was as good as it got for the Hornets who allowed Spurs to keep hold of the ball for much of the first half.
Davinson Sanchez was proving to be a threat in the air from set pieces, but the Colombia international did not make the most of a headed chance just before the break.
Spurs went onto the front foot after the restart with Christian Eriksen trying his luck with two long-range efforts, but it did not take long for the visitors to take a deserved lead.
Foster came and failed to connect with the ball and, after Lucas Moura was given the chance to get a cross into the middle, Doucoure chested the ball into the bottom corner from eight yards out with Foster still slightly out of position.
Spurs remained in the ascendancy but, as the game approached the final quarter, Watford began to build a head of steam and came agonisingly close to an equaliser when Alderweireld headed against his own bar from a Deeney cross.
Moments later, Watford had their goal. Roberto Pereyra's inswinging free kick found Deeney around 12 yards out, and the club stalwart was able to glance a header into the far bottom corner of the net.
Spurs had soon become second best in a game which they had largely controlled and seven minutes later, the North London outfit were behind as Cathcart got in front of his marker at a corner to power a header just inside of the post from eight yards out.
Mauricio Pochettino acted by throwing on Fernando Llorente but it was Harry Kane who should have brought his team level three minutes from time, only for the England frontman to head over from eight yards out.
Spurs were given a lifeline when five minutes of added-on time were indicated by the fourth official, but Watford held firm to extend what has been their best-ever start to a Premier League campaign.
WATFORD (4-4-2): Foster; Janmaat, Cathcart, Kabasele, Holebas; Hughes (Chalobah 86'), Doucoure, Capoue, Pereyra (Mariappa 90'); Gray (Success 70'), Deeney
TOTTENHAM HOTSPUR (3-4-2-1): Vorm; Alderweireld (Llorente 81'), Sanchez, Vertonghen; Trippier, Dembele (Winks 86'), Eriksen, Davies (Rose 89'); Alli, Lucas, Kane body check tags ::