0322 GMT July 19, 2018
The Reds, without Barcelona target Philippe Coutinho due to a back injury, fell behind to Stefano Okaka's header, BBC reported.
Sadio Mane equalized 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 equalize.
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 leveler.
On Friday night, Substitute Olivier Giroud headed in a dramatic late winner as Arsenal fought back to beat Leicester City 4-3 in a thrilling start to the 2017-18 Premier League season.
Arsène Wenger expressed his relief that Giroud, widely considered this summer to be either on the move or a player destined to be not much more than a super sub, remains very much at the heart of life at Arsenal, the Guardian reported.
The Frenchman’s thunderous header completed a thrilling comeback for Arsenal, and the idea that any suitors might lure Giroud away was met by “a determined no” from Wenger.
“He’s fantastic,” the manager added. Giroud had been linked with an exit during the summer, and Wenger admitted he considered allowing his most authentic target man to leave but the 30-year-old resisted the idea.
“I opened the door for him at some stage as I had many strikers and it’s important for him to play but I don’t want him to go. In the end he decided to stay and it was one of the great days of the recent weeks for me. I know how important he is not only as a player – he’s a fantastic player – but also in the group he has a big weight. It was a relief for me he decided to stay. Giroud in his prime is at the top of his game.”
The arrival of Alexandre Lacazette for a club record fee was supposed to spell the endgame in north London for a player who has divided opinion, but Wenger was pleased to imagine room for both – plus a handful more – in his attacking crew.
Lacazette scored on his debut against Leicester City, but Arsenal needed four goals in total to win a wild game shaped as much by loose defending as attacking brio.