Millwall manager Neil Harris defended goalkeeper David Martin after he was left "devastated" by his last-gasp error which led to Brighton completing a stunning comeback and reaching the FA Cup semi-finals with penalty shoot-out victory.
The Lions were two goals ahead with two minutes remaining after Alex Pearce's header and an Aiden O'Brien goal had put them on course for another Premier League scalp at the Den.
But Jurgen Locadia reduced the arrears with a fine strike two minutes before full-time, with Martin then flapping at a last-gasp Solly March free-kick as the ball dropped in to level the tie at 2-2 and send it to extra-time.
There were minimal chances during the 30 minutes of extra-time, although Millwall saw Shane Ferguson sent off late on for a stamp on Lewis Dunk as the game went to penalties.
Brighton striker Glenn Murray smashed the crossbar with the first spot-kick as Millwall converted their first three efforts only to see Brighton goalkeeper Mat Ryan keep out Mahlon Romeo's penalty with his foot.
Dale Stephens and Steve Morison then scored for their respective sides to take the shoot-out into sudden death, where Jake Cooper blazed over the sixth spot-kick to send Brighton to a Wembley semi-final against Manchester City.
Harris was seconds away from taking his side to the final four and said he had sympathy with Martin – who he revealed felt he had let the team down.
"What an outstanding performance for 94 minutes," he said.
"First and foremost I feel for David Martin because he was excellent throughout the game, absolutely commanded his box, made some good saves, good kicking and has made a slight error in judgement and it has cost us.
"He is a top pro, one of the nicest guys I've ever come across in football. He is obviously devastated. He has held his hands up and made a mistake.
"He said he let the boys down, he hasn't let the boys down. He made an error of judgement and I was more disappointed with the first goal, the defending wasn't good enough."
Brighton appeared on course to become the sixth-straight Premier League side to leave the Den with defeat – but manager Chris Hughton praised the spirit of his squad to complete a remarkable turnaround.
"The first word that comes to mind once you have won it is 'character'," he said.
"Credit to Millwall, what they wanted to do today they did very well. Although we had a lot of possession, they restricted us to minimal chances.
"At 2-0 down you are out of the competition, I felt we could get one back with the balance of play.
"But to get two back in that last period shows great character and of course missing the first penalty and them scoring the first three, also shows wonderful character and composure from a group of players that desperately wanted to go through."
Brighton last faced Manchester City in the FA Cup in 1983 – winning en route to their only previous appearance at this stage of the competition in the club's history.
The Seagulls made it all the way to the final that year before losing a replay to Manchester United, with Hughton admitting he was keen to avoid Pep Guardiola's City this time around. Watford meet Wolves in the other semi-final.
"We have a lot of people associated with this club that have been through the more difficult times," he added.
"It means everything. When you are a club the level of Brighton it is very difficult to get to this stage of the competition.
"You know it is not going to come round too often so when it does you have to big up the occasion as much as possible and praise the one who got us there. We will try to enjoy it as best we can.
"I don't think I'd be saying anything out of turn if all three of us (Brighton, Watford and Wolves) all would have wanted to avoid Man City.
"At this moment they are one of the best teams in the world and going for four competitions but, as shown today and yesterday, teams can get close and there can be upsets."
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