Villarreal's Adrian Lopez produced a stinging sucker punch in stoppage time to leave Liverpool reeling and move the Spaniards closer to a maiden European final after their Europa League semi-final first leg clash finished 1-0 on 28 April.
Liverpool, who got a taste of their own medicine having scored deep in added time to knock Borussia Dortmund out in the quarter-finals, looked comfortable before Denis Suarez broke into the box and unselfishly squared for Lopez to sweep home. The last-minute breakthrough sent the Villarreal fans into a frenzy.
Five-times European champions Liverpool had not lost in 12 Europa League games this season and were facing a Villarreal side out to banish a last-four curse and reach a European final for the first time at their fourth attempt. The Merseysiders' manager Jurgen Klopp downplayed the result, pointing out there was still a second leg to play at Anfield.
"To be honest it is the first leg and my first thinking when everybody was celebrating around me I thought: sorry but it's not over, you have to come to Anfield too. So we will be ready," he said.
"We knew it before that they are strong but I'm really happy with, because everybody knows, we started with something like high energetic football but it's not that we want to run like rabbits all the time. Today we need really to be patient.
"We played football, we opened the wing and we played really between the lines what we had to do against this team so that's it and that's what we did and so I was pleased with this, I didn't want to concede a goal but we did. That's the situation."
Klopp decided to leave the most prolific marksman at his disposal, Daniel Sturridge, on the bench for the entire 90 minutes and in his absence the visitors looked cautious and laboured in attack. They had their chances, with Joe Allen firing straight at keeper Sergio Asenjo from close range after five minutes while Roberto Firmino hit the post midway through the second half.
Villarreal, however, had chances of their own with Liverpool's Simon Mignolet making fine saves to deny Tomas Pina and Bakambu, who also hit the post with a header.
The winner arrived after Liverpool looked to have absorbed everything the hosts could throw at them and Lopez's late goal has left Klopp's side facing a tricky task next week, where an away goal would leave them needing to win by at least two. The second leg will be played at Anfield on 5 May.