Chelsea exploited United's draw at Sunderland with an important, hard-fought victory over Arsenal at Stamford Bridge.
Carlo Ancelotti's side moved ahead against the run of play when Didier Drogba neatly back-heeled home Ashley Cole's cross, and a thumping late free-kick from Alex sealed the victory for the champions.
The Blues are now five points clear of United, who were also overtaken on Sunday by local rivals Manchester City.
Roberto Mancini's Citizens overcame Newcastle in an attritional match at Eastlands. Carlos Tevez's penalty was cancelled out by fellow Argentine Jonas Gutierrez, before Adam Johnson's second-half strike settled a match marred by a suspected