The Reds, without Barcelona target Philippe Coutinho due to a back injury, fell behind to Stefano Okaka’s header.
Sadio Mane equalised with a clinical finish only for Abdoulaye Doucoure to restore the lead from close range.
Roberto Firmino’s penalty made it 2-2 before Mohamed Salah thought he had won it only for Britos to equalise.
In Jurgen Klopp’s 100th competitive game as Reds boss, Liverpool wasted a string of chances before Uruguay defender Britos headed over the line from close range.
Watford keeper Heurelho Gomes, who conceded the penalty after hauling down Salah, denied Alberto Moreno with a fingertip save.
Joel Matip also hit the bar and Dejan Lovren forced another good save from Gomes as the Reds dominated the closing stages until Watford’s late leveller.
Watch and download Watford Vs Liverpool Highlights EPL Match Day 1 below: –