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 on Boxing Day.
With nine points available in less than a week the leading duo offered more proof that they appear to be 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 illustrated leaders Chelsea's superiority over a West Ham side that reached Christmas in the Champions League places.
The derby win briefly put them six points clear with 45 from 18 matches but City replied 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 derby in two spells as Chelsea manager.
Manchester 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 netting.
West Ham dropped to fifth, one point ahead of Arsenal and Tottenham Hotspur who both enjoyed narrow wins.
Arsenal had striker Olivier Giroud sent off but held on to beat Queens Park Rangers 2-1 with goals from Alexis Sanchez, who also missed a penalty, and Tomas Rosicky. Charlie Austin replied from the penalty spot for QPR.
Tottenham had goals from Harry Kane and Christian Eriksen to thank for a 2-1 win at bottom club Leicester City. Swansea City are eighth after beating Aston Villa 1-0.
Raheem Sterling's goal gave Liverpool a 1-0 victory at Burnley but Everton's troubles continued as they lost 1-0 at home to Stoke City.
While plenty of clubs will enter the New Year eyeing a top-four place, Chelsea and City are set to scrap for the title.
Chelsea produced a display full of power, pace and skill against a West Ham side that 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 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 showed his eye for goal 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 hit the post.
Manchester City responded emphatically and were out of sight before halftime at West Brom.
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.
