The visitors doubled their lead when Sanchez dinked over Heurelho Gomez after Theo Walcott’s low cross; the ball adjudged to have crossed the line by the Goal Decision System.
Mesut Ozil gave his side a three goal advantage at half time when he headed home the Chile international’s cross.
Watford debutant Roberto Pereyra, on as a half time substitute, reduced the deficit with his left foot on 57 minutes.
The Hornets continued to press in the latter stages, however, the Gunners held firm to maintain their 100 per cent Premier League record against their opponents.
See: Watford match report
See: Arsenal match report