Premier League champions Manchester United were thrashed 3-0 at Fulham on Saturday while neighbours Manchester City sacked manager Mark Hughes and brought in Italian Roberto Mancini.
Goals by Danny Murphy, Bobby Zamora and Damien Duff condemned second-placed United to a second league defeat in eight days. Chelsea can now stretch their lead to six points by beating struggling West Ham United on Sunday.
A second win in 11 league matches proved in vain for Hughes who watched his City side beat Sunderland 4-3 at Eastlands before being dismissed after the game.
A day of surprises began with Liverpool's 2-0 reverse at bottom club Portsmouth, their seventh league defeat this season.
Arsenal are third, two points behind United, after a 3-0 win over Hull City. Aston Villa occupy fourth spot after a 1-0 victory over Stoke City.
Tottenham Hotspur, in fifth, followed up their midweek demolition of Manchester City with a 2-0 victory at Blackburn Rovers.
United, fielding a makeshift defence because of a string of injuries, went behind after 22 minutes when midfielder Murphy seized possession from Paul Scholes and fired past goalkeeper Tomasz Kuszczak from long range.
In the 46th minute in-form striker Zamora met a Clint Dempsey knockdown to half volley past Kuszczak and notch his 10th goal of the season.
United manager Alex Ferguson threw on attacking substitutes Dimitar Berbatov and Danny Welbeck but a superb Duff volley with 15 minutes remaining sealed an emphatic victory for the hosts.
"We were playing a United side suffering very badly from injuries but we can't do much more I suppose than play as well as we can play and get the best result we can get," Fulham manager Roy Hodgson told Sky Sports.
Hodgson gave special praise to striker Zamora who has bagged five goals in four games.
"He is playing very well. He is a handful for any defence. I think he would have been a handful if they had (first-choice central defenders) Rio Ferdinand and Nemanja Vidic playing," he said.
Manchester City are sixth after their thrilling win over Sunderland who had Michael Turner sent off in stoppage time.
Craig Bellamy set up Roque Santa Cruz for his first league goal for City before Carlos Tevez converted a penalty to make it 2-0 after 12 minutes.
Sunderland hit back to level through defender John Mensah and a rocket shot from Jordan Henderson, only for Bellamy to curl home City's third on 35 minutes.
Kenwyne Jones then headed Sunderland level again before Santa Cruz grabbed his second with 21 minutes to go.
City, soccer's richest club, now welcome former Inter Milan coach Mancini, with former Blackburn manager Brian Kidd joining as his assistant.
"A return of two wins in 11 Premier League games is clearly not in line with the targets that were agreed and set," chairman Khaldoon Al Mubarak said in a statement.
A lacklustre Liverpool lost at Portsmouth as goals by Nadir Belhadj and Frederic Piquionne heaped more pressure on manager Rafa Benitez.
Brazilian midfielder Denilson's delightful free kick set Arsenal on the way to victory but they had to survive a Geovanni missed penalty after the break before adding two more goals through Eduardo da Silva and Abou Diaby.
A header from Norwegian frontman John Carew sealed Villa's win over Stoke while Peter Crouch's double helped Tottenham sink Blackburn.