Dimitar Berbatov fired Manchester United level on points with Manchester City as his hat-trick inspired a 5-0 win over 10-man Wigan, while the Premier League leaders were held to a 0-0 draw at West Bromwich Albion on Monday.
Berbatov has been a bit-part player for long periods this season but the Bulgarian striker seized a chance to replace the rested Wayne Rooney with a clinical treble and futher goals from Park Ji-Sung and Antonio Valencia destroyed third bottom Wigan.
United's five-goal spree -- their second in six days after a 5-0 win over Fulham -- means City are now top only on goal difference after Roberto Mancini's side suffered a rare setback at the Hawthorns.
City, who were five points clear of United earlier this month, enjoyed most of the possession, but couldn't find a way past Albion goalkeeper Ben Foster with Mario Balotelli hitting the woodwork and also having a goal disallowed.
"Today was difficult," Mancini said. "When you a play a team with all players behind the ball, if you don't score in the first half it's not easy, you don't have any space. Maybe we were too slow in the first half."
At Old Trafford, Sir Alex Ferguson played midfielders Valencia and Michael Carrick in defence, but Park put the hosts ahead in the eighth minute when the South Korea midfielder converted Patrice Evra's cross.
United got another boost when Wigan striker Conor Sammon was harshly dismissed for an elbow on Carrick just before half-time.
The champions took full advantage as Berbatov increased their lead with a clinical finish in the 41st minute.
Berbatov added his third goal in two games with another cool strike from Valencia's pass in the 58th minute.
Former Wigan star Valencia struck in the 75th minute and Berbatov completed his treble from the penalty spot three minutes later after Park was tripped by Antolin Alcara.
Ferguson conceded United got a lucky break with Sammon's red card.
"I thought it was a softish one in that I don't think the boy acted intentionally. It was a bit harsh, I have to say that," he said.
At Stamford Bridge, Chelsea's title hopes suffered another major blow as Clint Dempsey's equaliser gave Fulham a 1-1 draw against their west London neighbours.
A third successive draw means even a place in the top four can't be taken for granted as Chelsea will slip to fifth if Arsenal beat Wolves on Tuesday.
Andre Villas-Boas's spluttering side took the lead two minutes into the second period through Juan Mata but Dempsey levelled in the 56th minute.
"The title will be pretty difficult now," Villas-Boas admitted.
"It would have been a good December for us if we'd finished with six points from two home games, but taking this draw into account it makes it difficult now. You have to be realistic."
Liverpool's challenge for a Champions League place was dented as struggling Blackburn earned a 1-1 draw an Anfield.
Rovers, beaten in their previous three matches, took the lead in first half stoppage time when Reds midfielder Charlie Adam deflected Morten Gamst Pedersen's corner into his own net.
Maxi Rodriguez equalised with a header from Martin Skrtel's flick in the 53rd minute and Reds boss Kenny Dalglish sent on England midfielder Steven Gerrard, finally back from an ankle injury, for the final 20 minutes.
"The chances we create are good, but we can't keep saying we have come up against an inspired goalkeeper," Dalglish said.
Newcastle surged back into the race for a top four place with a 2-0 win at struggling Bolton.
Alan Pardew's side had failed to win any of their previous six matches but they got back on track to move within four points of Chelsea thanks to second half goals from Hatem Ben Arfa and Demba Ba.
Bolton remain second bottom after their sixth defeat in seven matches.
Sunderland's revival under new boss Martin O'Neill was hit by a controversial 1-1 draw against Everton at the Stadium of Light.
O'Neill's side, who had won two of their three matches since the former Aston Villa boss took charge, went ahead through Jack Colback's deflected shot in the 26th minute.
However, Everton earned a controversial equaliser in the 51st minute when referee Howard Webb awarded a penalty after Leon Osman stumbled while shooting under no challenge and Leighton Baines blasted in the spot-kick.
In the day's late match, Stoke drew 0-0 against Aston Villa after City defender Marc Wilson hit the bar in the second half.