Chelsea replaced Liverpool at the top of the Premier League with a comfortable 1-0 win at Middlesbrough on Sunday.
It was the London side’s sixth successive league victory without conceding a goal and put them on 28 points from 12 games, a point ahead of Liverpool and Manchester City.
Diego Costa hit his 10th goal of the season to put Chelsea in front four minutes before the break.
Middlesbrough found it hard to make any clear opportunities against a solid Chelsea defence and remain in the bottom six, one point clear of the relegation places.
“We knew it was going to be hard but we are in the Premier League and one mistake gets you punished,” Middlesbrough manager Aitor Karanka said.
Middlesbrough have now lost their last seven league and cup games against Chelsea without scoring and have not beaten them since 2006.
“Confidence and spirit are up and we are playing well,” Chelsea defender Gary Cahill said.
“We are getting the results too which is the most important thing. We have carried on from where we left off at the international break.
“We are happy with today’s work, but the league is not won today. It is not even Christmas yet.”