Chelsea forward, Tammy Abraham, opened scoring five minutes into the match against Watford when Jorginho’s delicious pass allowed him to lift in his 10th goal of the season.
Foster made two superb stops to prevent Frank Lampard’s side going further ahead before the break as he denied both Christian Pulisic and Mason Mount.
But he could not prevent Pulisic following up his hat-trick against Burnley last Saturday with another goal as the American tucked in Abraham’s low cross.
Watford, who remain bottom of the Premier League and are still searching for their first league win of the season, were handed a lifeline when Gerard Deulofeu tucked in a video assistant referee-awarded penalty, his first goal of the season.
And the hosts almost equalised when Foster’s header deep in injury time was superbly saved by Chelsea goalkeeper Kepa Arrizabalaga, who sparked celebrations among his team-mates with the last action of the match.
But overall, Watford could not match a superior Chelsea side who recorded a fifth successive away win in the league, and a club-record equalling seventh away victory in all competitions.
Lampard’s side can slip back into fourth behind Leicester, who face Crystal Palace on Sunday and have a superior goal difference, yet it was another impressive step forward for his young side who are now two points behind second-place Manchester City.