Liverpool travelled to Vicarage Road under the good omen of having won their last four Premier League openers, while Watford's last opening win came in 1987.
However, the home side looked to get their first opening day win in the Premier League as they cancelled out Liverpool, looking much the better side in the early minutes.
Okaka worked well to offer Pereyra the first chance of the game and the winger's effort was pushed wide by Mignolet. The Reds seemed to have cleared the pressure, but the resulting corner kick saw the home side take the lead to get the crowd bouncing.
The corner was whipped in from the right and the Liverpool defence failed to pick up Okaka's run. The Watford big man powered a free header at goal which could not be kept out by Mignolet despite getting a hand to it.
Klopp's side were unable to respond with Watford in control, but a flash of brilliance from Sadio Mane around the half-hour mark drew the game level with Liverpool's first shot on goal.
The Senegalese winger initiated the passing move, before Moreno found Can who flicked an inch-perfect ball into the path of Mane. With only Gomes to beat, the former Southampton man curled the ball into the top far corner.
Just as it looked like they were back in the game, Watford punished another defensive gift from the Reds. Cleverley provided the low cross from right and the sliding Liverpool defenders could not clear the ball which dropped for Doucouré a few yards out. The midfielder made no mistake and smashed home from close range.
Salah managed to find some space and fashioned himself a couple of chances at the end of the first half and in the early minutes of the second, and in the 54th minute the Egyptian was clipped by Gomes inside the box.
The penalty looked clear and Firmino stepped up to send the keeper the wrong way and find the bottom corner. Three minutes later, Liverpool fans were sent into delirium as new signing Mohamed Salah scored his first Premier League goal with Liverpool to seal the turnaround.
Lovren's long ball released Firmino behind the back four and his lobbed finish beat Gomes but was drifting wide before Salah raced to get on the end of it and finish into an empty net.
The game opened up with Klopp's side looking to put the match out of reach while Watford refused to give up and drove forward in search of an equaliser.
Matip's header hit the crossbar with Gomes already beaten while Moreno's volley was superbly tipped over the bar by Gomes. Minutes later, Lovren's header was then saved by the keeper.
The Reds failed to put the game to bed as the fixture entered a dangerous stage. Watford began to put the pace into their attacks for a late charge, and the crowd got to their feet in support as the assistant referee held up the board signalling five minutes were to be added on.
And, incredibly, Watford found their reward. Liverpool once again failed to clear a corner and Richarlison blasted a strike towards goal from the byline. The ball ricocheted off Mignolet's arm and onto the underside of the crossbar before Britos reacted quickest to bundle it over the line.
As the new season kicks off, questions will be raised regarding Liverpool's defence as well as their uneven performance. Watford secure a merited point in their attempt to prove wrong those who favour them to be relegated this season.