Arsenal's Premier League title bid was left in tatters as they failed to recover from Watford's early double blow in the visitors' shock 2-1 win at Emirates Stadium.
The Hornets, who were winless in seven league games, made an incredible start to race into a 2-0 lead inside 13 minutes on Tuesday, with two former Tottenham players central to Arsenal's downfall.
Younes Kaboul's deflected free-kick opened the scoring, while Etienne Capoue pounced on Gabriel's loose throw before venturing forward unchallenged and drawing a save from Petr Cech, from which Troy Deeney pounced on the rebound.
It should have served as a wake-up call to Arsenal, for whom manager Arsene Wenger is still serving a touchline ban, but it was the visitors who remained on the front foot and they saw efforts from Sebastian Prodl and Miguel Britos repelled before the end of a first half in which Aaron Ramsey was forced off injured.