The Switzerland international midfielder, formerly with West Ham, has not had a medical ahead of the planned move to Vicarage Road yet.
But Hamburg have told the German media that Behrami has left pre-season training in order to further negotiations.
The 30-year-old joined Hamburg from Napoli last summer for £4.3m but reports suggest Watford will be able to acquire him for around half that fee.
Behrami joined West Ham from Lazio in the summer of 2008 and went on to make over 50 appearances for the Hammers before returning to Italy to join Fiorentina.
Meanwhile, Tottenham midfielder Benjamin Stambouli is still considering a move to Watford after the clubs agreed a fee around a week ago.