Chelsea overpowered West Ham United 2-0 and Manchester City outclassed West Bromwich Albion 3-1 as the Premier League title rivals flexed their muscles at the start of a hectic holiday programme.
With nine points available in less than a week the leading duo provided further evidence that they are in a mini-league of their own, although third-placed Manchester United's resurgence continued with a 3-1 defeat of Newcastle United.
Goals from John Terry and Diego Costa hardly did justice to leaders Chelsea's superiority over a West Ham side that reached Christmas in the Champions League places.
The derby win briefly put Chelsea six points clear with 45 from 18 matches but City responded to close the gap to three with a seventh successive league win -- Fernando, Yaya Toure (penalty) and David Silva on target before halftime at a snowy Hawthorns.
"Football is football, you never know and the only thing we can do to try to reduce the unpredictability of football is to do what we're doing: playing very well," said Jose Mourinho who is yet to lose a home London derby in two spells as Chelsea manager.
United are 10 points off the pace but growing in confidence after Wayne Rooney scored twice and Robin van Persie once to see off Newcastle at Old Trafford.
Southampton's return to form was underlined by a 3-1 win at Crystal Palace which lifted them back into the top four.
Sadio Mane, Ryan Bertrand and Toby Alderweireld scored to give Southampton a second successive victory after a run of four straight defeats for Ronald Koeman's side.
West Ham dropped to fifth, one point ahead of Tottenham who had goals from Harry Kane and Christian Eriksen to thank for a 2-1 victory at bottom club Leicester City.
Swansea City are seventh after beating Aston Villa 1-0.
Arsenal can return to the top six if they beat Queens Park Rangers in the day's late kickoff.
Raheem Sterling's goal gave Liverpool a 1-0 win at Burnley but Merseyside rivals Everton's troubles continued as they slumped to a 1-0 home defeat by Stoke City.
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While plenty of clubs will enter the New Year eyeing a top-four place, Chelsea and City are likely to scrap for the title.
Chelsea produced a display full of power, pace and skill against a West Ham side who managed a 0-0 draw in the corresponding fixture last season, prompting Mourinho to accuse their opponents of 19th Century football.
The first half hour was one-way traffic and the only surprise was that Mourinho's men did not score before Terry turned the ball over the line after Costa flicked on a Cesc Fabregas corner.
It was the Chelsea captain's second goal in successive league games.
Costa, whose goal tally has slowed in recent weeks, showed his class when he received a pass from the excellent Eden Hazard just past the hour and lashed a left-footer past goalkeeper Adrian.
Chelsea had numerous other chances to score with West Ham's only flurry coming near the end when substitute Morgan Amalfitano dinked an effort against the post.
City responded emphatically and were out of sight before halftime at West Brom.
Home keeper Ben Foster dropped the ball at the feet of Fernando after eight minutes to gift the visitors their opener and Toure's spot-kick made it 2-0. Silva stroked in the third after 34 minutes to end the game as a contest.
Torrential snow slowed City's slick passing after the break and Brown Ideye grabbed a consolation goal for WBA.
"I am very pleased with the performance. It was very difficult to play especially in the second half," said City boss Manuel Pellegrini.
Elsewhere, Hull City ended a run of 10 league games without a win by beating Sunderland 3-1 to move out of the bottom three.
