Dirk Kuyt grabs the late winner at Anfield to put home side through.
Liverpool send Manchester United out of FA Cup
It was a sweet moment for Kenny Dalglish's side after their win over City in the Carling Cup semi-finals in midweek and against a background of heightened tensions between the two bitter rivals following Luis Suarez's eight-match ban for racially abusing United defender Patrice Evra.
Daniel Agger opened the scoring with a 21st-minute header for the hosts but Park Ji-sung equalised six minutes before the break and a replay looked on the cards until Kuyt grabbed the winner two minutes from time.
Kenny Dalglish, the Liverpool manager, praised the attitude of players and fans alike at Anfield.
There were few flashpoints in the game, as Liverpool made it two cup victories in three days, having reached the Carling Cup final in midweek at the expense of Manchester City.
“It was a fantastic effort from our players after what they had to get through on Wednesday night,” Dalglish said.
“Their commitment, attitude and desire to get a result was fantastic and we cannot speak highly enough of them after that performance and result today.”
Evra was booed throughout by Liverpool fans but nothing untoward happened inside the ground. “I think both sets of fans were a magnificent advert for their clubs,” Dalglish said.
Sir Alex Ferguson was unable to mask the shattering disappointment of losing a game he did not think it was possible United would fail in, once Park had levelled.
“To lose the game is hard to believe,” said the United manager. “We dominated the game and we were comfortable. Our movement off the ball was good and I thought we were in the driving seat.”
Indeed, so much in command did Ferguson think the visitors were yesterday that he introduced Javier Hernandez in an effort to avoid a replay. “I was looking to win it,” he said. “To lose it is a real sickening blow.”