Chelsea won at Watford thanks to Hakim Ziyech’s goal 18 minutes from time, keeping them top of the Premier League table.
Mason Mount’s clinical finish put the Blues ahead after Alonso drew the ball back for Havertz, who squared the ball for the England international to slot home.
Moussa Sissoko put Emmanuel Dennis through on goal two minutes before halftime, and he calmly waited for Antonio Rudiger to confront him before passing the ball through his legs and into the net.
Following a superb pullback from Mount, second-half substitute Ziyech clinched the game with a rocket into the top of the net.
From the first whistle, Watford were determined not to allow Chelsea rest in control, and Mendy had to make a wonderful save from Azpilicueta’s inadvertent volley from Kiko’s cross.
The game was delayed for nearly 30 minutes while a spectator who had had a heart arrest was stabilized.
Watford kept up the pressure after the restart, however, Mount’s half-volley latching onto Ruben Loftus-header Cheek’s over the defence was prevented by the width of the post.
In the dying stages, the Hornets battled for an equaliser, with Mendy having to tip over a late rocket free-kick from Juraj Kucka and last-ditch challenges being necessary to prevent more attempts on goal.