Arsenal scored twice in the last nine minutes to overcome in-form Watford and record their seventh straight win in all competitions.
With 10 minutes to go it appeared that fourth-placed Hornets, with wins over Tottenham and Manchester United this season, would also frustrate another Premier League giant.
But that was until the introduction of Alex Iwobi midway through the second half. The Nigerians driven low ball from the right resulted in Arsenals opener, inadvertently turned in by Watfords Craig Cathcart.
Two minutes later Iwobi was involved in a one-two with Alexandre Lacazette, who delivered a low cross for Mesut Ozil to sidefoot home.
With the game goalless, Arsenal substitute keeper Bernd Leno made a great one-handed save to deny Andre Gray, while Issac Success fired narrowly wide of the far post.
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