Liverpool forward Dirk Kuyt's hat-trick dealt a body blow to Manchester United's Premier League title hopes as the leaders were beaten 3-1 at Anfield on Sunday.
United suffered their second league defeat in a week after losing 2-1 at Chelsea on Tuesday, leaving them just three points ahead of Arsenal having played a game more.
Kuyt opened the scoring after 34 minutes from point-blank range after some trickery by Luis Suarez and the workaholic Dutchman doubled Liverpool's lead five minutes later after a terrible mistake by the unfortunate Nani.
Kuyt put the game beyond United midway through the second half, again with a simple finish, after keeper Edwin van der Sar failed to hold a curling Suarez free kick.
United substitute Javier Hernandez scored a consolation in stoppage time.
United have 60 points from 29 games. Arsenal have 57 from 28 with third-placed Manchester City on 53 from 29. Liverpool are sixth on 42 points.
Liverpool's dreadful early-season long since ended their own hopes of a title challenge but, after a stirring rendition of their anthem, set about trying to put a spanner in the works of United's charge towards a record 19th English title.
Uruguayan Suarez, a deadline day signing who plays very much in the image of Liverpool manager Kenny Dalglish, was clearly determined to make an immediate impact on his first taste of the biggest derby in English football.
He buzzed around from the start and had an early half chance as Liverpool set a high tempo.
However, United's more measured approach gradually got them into the game and Dimitar Berbatov came closest to opening the scoring with a shot that clipped the woodwork.
An absorbing contest played in bright sunshine caught fire as halftime approached.
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