Champions United overwhelmed Boro 4-1 at Old Trafford to go top temporarily, while Chelsea destroyed City 6-0 to close the gap on the hapless Citizens.
At Old Trafford, the home side needed three minutes to take the lead when Portuguese winger Luis Nani fired in the opener.
However, Boro, who were missing the services of their Egyptian forward Ahmed Hossam 'Mido', swiftly leveled the scoring three minutes later through Jeremie Aliadiere.
In-form England striker Wayne Rooney restored the Red Devils' lead on 33 minutes, before Carlos Tevez grabbed a second-half brace in the 55th and 85th minutes to complete the rout.
United claim top spot with 26 points from 11 games, while Boro remain on eight points, one point off the relegation zone.
Chelsea goal feast
At Stamford Bridge, Michael Essien opened the scoring for the home side after 16 minutes, as the Blues' top marksman Didier Drogba doubled the lead in the 31st minute.
The on-fire Drogba hit his second of the game 11 minutes into the second half, Joe Cole made it four on the hour mark, before Salomon Kalou added a fifth on 75 minutes.
Substitute Andrey Shevchenko completed the drubbing with a sixth goal in injury time, to destroy Sven Goran Eriksson's men, who just conceded seven goals in total before this game.
The Blues reached 21 points to close the gap with City to one point.