The visitors twice fell behind at Vicarage Road, but goals from each of Jürgen Klopp’s front three - Sadio Mane, Roberto Firmino and Mohamed Salah - appeared to have been enough to earn a hard-fought victory for the visitors.
However, Miguel Britos netted in injury time to ensure Watford claimed a share of the spoils.
Mane’s fine strike equalised Stefano Okaka’s early opener, but Watford regained their lead before half-time through Abdoulaye Doucoure.
Two goals in the space of two minutes shortly after the restart - the first a Firmino penalty and the second a poacher’s effort from competitive debutant Salah - turned the match on its head, only for Britos to level deep into stoppage time.