Watford substitute Mileta Rajovic’s late goal secured a dramatic win against Millwall as goalkeeper Matija Sarkic was remembered at The Den.
Edo Kayembe’s direct corner and Giorgi Chakvetadze’s brilliant free-kick had given Watford a two-goal lead.
Duncan Watmore’s second-half double looked to have rescued a point for the home side, who had seen earlier efforts from George Honeyman and George Saville hit the post and a strong penalty shout waved away.
But barely a minute after the equaliser, Rajovic stole in to head past new Lions keeper Lukas Jensen.
Watford held firm for nine minutes of stoppage time, which were added on after a lengthy break after a clash of heads between Ryan Leonard and Mattie Pollock.
Watmore had the chance to level things up again and complete a hat-trick, but he could not find a way past Watford goalkeeper Daniel Bachmann.
New Millwall signing Macauley Langstaff came off the bench in the second half and hit the crossbar in the final moments, albeit from an offside position, as the Hornets held on for a opening-day victory.
Fitting tributes to Sarkic
Among the excitement of a new season at The Den, there was a more poignant message as the club remembered Sarkic, who died in his home country of Montenegro in June at the age of 26.
The Montenegro international was only with the Lions for a year, but he cemented himself as the number one for both Joe Edwards and then Neil Harris as the club finished the season in 13th.
"That was the key part of today. Matija's family were here and for us to be able to pay tribute to him and remember him with a Millwall send-off, which both sets of fans did, for us as human beings, what a privilege to be able to be here and do our jobs," Harris told BBC Radio London.
There were a number of tributes to Sarkic before the game, including wreaths being brought out and laid behind both goals and a touching video on the big screen.
Sarkic’s family were welcomed on to the pitch in a highly emotional moment, with tears in the crowd and also a banner carrying the message "RIP Matija Sarkic" displayed by the travelling Watford fans.
A tree has been placed outside the players' entrance to the stadium and a mural unveiled in the Cold Blow Lane end.
What the managers said
Millwall boss Neil Harris told BBC Radio London:
"We played with too much emotion, certainly in the first 25 minutes, and made really poor decisions with the ball.
"We conceded a really poor goal, it's a goal that doesn't belong in under-12s football, let along first-team football at The Den.
"I thought we were exceptional in the second half and again Watford's first venture into the final third ends up with the ball in the net."
Watford boss Tom Cleverley:
"They came out in the second half and can probably feel a bit hard done by when we scored the second goal when they were on top. We showed great character then to go 2-0 up and huge character at the end."
On Chakvetadze: "Quality performance. Three games in pre-season and then today, he's in a really good moment.
"He's someone I enjoy watching train and play, it was a beautiful strike and a really good all-round performance."