The Reds took the lead after 21 minutes when Daniel Agger bulleted a header into the back of the net from a Steven Gerrard in-swinging corner.

But United equalised with six minutes of the first half remaining via Ji-Sung Park's powerful low volley from Rafael da Silva's centre.

Both sides pushed for a winner in the second period, but it was Liverpool who prevailed when Kuyt latched onto Andy Carroll's flick and drilled home at the Kop end.

Following Wednesday night's pulsating Carling Cup semi-final success over United's neighbours City, Kenny Dalglish made four changes to his line-up.

Jamie Carragher came into the starting XI on his 34^th birthday to operate in a holding midfield role. Carroll, Maxi Rodriguez and Martin Kelly were also restored in place of Charlie Adam, Glen Johnson, Dirk Kuyt and Craig Bellamy.

With one Wembley appearance now secured, the game kicked-off with Liverpool aiming to take a step towards another cup final in what was the pick of the FA Cup fourth round ties.

Four minutes were on the clock when Maxi fired the first effort of the contest with his stinging drive from the edge of the area palmed around the post by David de Gea at full stretch. At the other end, Ryan Giggs's volley from Park's knockdown was easily gathered by Pepe Reina.

The game had begun in cagey fashion as both teams set about attempting to gain control of the contest, but on 18 minutes Antonio Valencia saw an effort cannon off the base of the upright after the United winger had carried the ball to the edge of the area.

However, it was Liverpool who made the breakthrough on 21 minutes courtesy of an Agger header.

The Dane rose highest amongst a crowded area to meet Gerrard's corner and divert the ball into the back of the net via his forehead, with United 'keeper De Gea caught out of position.

It was Agger's first competitive goal since netting against Benfica in April 2010.

With their tails up, Gerrard then collected Carroll's layoff and surged goalwards before striking over the bar soon after.

But United drew level six minutes before half-time when Rafael held off the challenge of Jose Enrique and crossed from the right byline to allow Park to hook an effort into the bottom corner and ensure the teams headed back down the tunnel on level terms.

The opening stages of the second half continued in a similar manner to the first, with clear chances remaining hard to come by for both sides.

Liverpool appealed for a penalty 11 minutes after the re-start, claiming Chris Smalling's attempt to cut out Enrique's pass to Carroll had struck the hand of the United defender, but referee Mark Halsey waved play on.

The Reds then had Martin Skrtel to thank for preventing the visitors from going ahead when Danny Welbeck knocked the ball around the onrushing Reina on the edge of the box, but the defender was on hand to avert the danger.

Dalglish opted to make a double change just after the hour, with Kuyt and Adam replacing Maxi and Carragher.

With play now beginning to switch from end to end, Gerrard forced a save out of De Gea with a curling free-kick while Michael Carrick struck off target for United. It proved to be the skipper's last act as he made way for Bellamy soon after.

The final 10 minutes saw Liverpool press for a winner and it duly arrived on 88 minutes when Carroll's flick from a long ball sent Kuyt scampering into the box, and the Dutchman made no mistake as he dispatched a volley beyond De Gea to send the Reds into the fifth round.

Liverpoolfc.tv man of the match: Martin Skrtel