Manchester United wiped out Manchester City's lead at the top of the Premier League on Tuesday with a 2-0 victory over Stoke as its neighbour went down 1-0 at Everton to a goal by a former United player.
Darron Gibson, who left Old Trafford earlier in January, struck on the hour for Everton against City, which is now only top due to its superior goal difference over United.
Defending champion United scored both goals from the penalty spot through Javier Hernandez and Dimitar Berbatov.
Tottenham is five points off the lead after Gareth Bale's double helped the north London sweep past Wigan with a 3-1 win.
Chelsea is now seven points adrift of Spurs in fourth after requiring an own goal to salvage a 1-1 draw at Swansea.
Liverpool provisionally climbed above Arsenal into fifth place with a 3-0 win at Wolverhampton Wanderers, ahead of the Gunners' match against Bolton on Wednesday.
The Premier League's leading sides were in action as their officials were wrapping up deals in the final hours of the January transfer window.
Alex Ferguson's United side was without 11 senior players, including both first-choice goalkeepers, but had little trouble against a lackluster Stoke side that rarely threatened.
United took the lead after Park Ji-sung raced onto a short pass from Paul Scholes and was brought down by Jermaine Pennant.
Hernandez found the bottom corner in the 38th-minute spot kick, sending Thomas Sorensen the wrong way. But he handed over penalty duties to Berbatov seven minutes into the second half after Jon Walters had bundled Antonio Valencia over in the area.
"The first (penalty) one was very debatable," Stoke manager Tony Pulis. "The second one was [a penalty] ... and the one he didn't give - for a foul on Patrice Evra - was a definite penalty." The result at Goodison Park made it a perfect night for United.
Sir Alex Ferguson is eyeing the final Manchester derby as the key point in this season's Premier League title race.
"This was obviously a significant victory but we are only on the last day of January," he said.
"I always see March and April as the important month. We've got Manchester City in April for instance."
Roberto Mancini blamed his own complacency for his side's costly defeat at Everton.
The result prolonged a dismal record at Goodison Park for City, who have now won just once in their last 14 league visits to the ground.
Despite that unimpressive statistic - and with Mancini trailing 4-1 in his head-to-head record with opposite number David Moyes prior to the game - the Italian said he still underestimated the Merseysiders.
Mancini said: "I didn't prepare well for this game. I thought it was maybe easier. I know here it is never easy, never.
"I acknowledge my mistake. It is important for me to know this because, like this, I can't do the same mistakes in the next game.
"The players put everything on the pitch but I made some mistakes during the last three days preparing for this game."
At White Hart Lane, Tottenham manager Harry Redknapp went to the match straight from his tax evasion trial at a London court.
His side had little trouble dispensing of Wigan on another night when Bale shone for the north London club to consolidate its bid for an automatic Champions League qualification place.
Bale volleyed in a 29th-minute opener that was followed up by Luka Modric's cool finish five minutes later from 18 yards. Bale struck again for his 11th goal of the season in the 64th, before James McArthur scored a consolation goal for Wigan in the 80th.
In south Wales, a last-gasp own goal by Neil Taylor salvaged a point for Chelsea against Swansea, which beat Arsenal at home two weeks ago.
Former Chelsea winger Scott Sinclair had given Swansea the lead five minutes before halftime, volleying the ball over goalkeeper Petr Cech after a slack defensive header by Jose Bosingwa.
Chelsea's hopes of a comeback appeared over when defender Ashley Cole was sent off for a second booking in the 86th. However, the visitors leveled when Bosingwa's shot was deflected into the net by Taylor in injury time.
Fernando Torres failed to score his first Premier League goal for Chelsea since September on the first anniversary of his 50 million-pound move from Liverpool.
But the striker who joined Liverpool on that same day did enjoy a happy anniversary in the win at Wolves, with Andy Carroll striking his first league goal since October 29. Craig Bellamy added another in the 61st and Dirk Kuyt wrapped up Liverpool's second win in its past seven league matches in the 78th.
"It was a very professional, high-quality performance," Liverpool manager Kenny Dalglish said. "It was great for Andy to score a goal. Craig scored a fantastic goal, as did Dirk."