Costa strike sends Blues to summit
Diego Costa scored his 10th goal of the season as Chelsea won 1-0 at Middlesbrough to record a sixth consecutive victory and take over at the Premier League summit.
Costa struck for the third straight game with half-time approaching as Antonio Conte's side underlined their title credentials by posting a sixth clean sheet in a row.
It took them a point clear of Liverpool and Manchester City, and three ahead of fourth-place Arsenal.
Only the woodwork prevented Chelsea from winning by a more handsome margin as Pedro Rodriguez hit the bar midway through the second half, during which Middlesbrough were forced to hang on for long spells.
Aitor Karanka's side retained hope while the deficit remained at a single goal, but the gulf in class between the sides was clear and it leaves Boro one point above the relegation zone in their first season back in the Premier League following promotion.
Costa rewarded Chelsea's dominance as the Spaniard put the visitors -- unchanged for the fifth consecutive league game -- ahead four minutes before the end of a tight first half.
It stemmed from a disputed corner, as Middlesbrough claimed debutant full-back Fabio da Silva had not touched the ball as it ran out of play.
Eden Hazard's in-swinging set-piece looped into the air as Calum Chambers and Gaston Ramirez challenged Marcos Alonso at the near post.
Despite the hosts having numbers back to defend Costa reacted first to volley the loose ball home from inside the six-yard box, taking him to double figures for the campaign.
Middlesbrough goalkeeper Victor Valdes was partially at fault for failing to come off his line.
His indecision was in stark contrast to earlier in the half when the former Barcelona man produced a stunning fingertip save to deflect a first-time Pedro shot over the bar.
It was one of few clear-cut chances before the interval as Boro were largely successful in their attempts to negate the threat of Conte's side by packing midfield and looking to counter-attack.
The out-of-touch Alvaro Negredo wasted their two main openings, first firing across the face of goal and then heading wide from Antonio Barragan's deep cross.
Ramirez shot over early in the second half after Negredo's dummy created space on the edge of the box for the Uruguay international, but it was a rare threat from the hosts.
Boro have forced draws against Arsenal and Pep Guardiola's City this season, but after falling behind they rarely looked like troubling Chelsea's three-man back line.
Having not conceded a league goal since September 24 -- almost 10 hours of football -- Chelsea are on their best defensive run for six years.
Goalkeeper Thibaut Courtois preserved their latest shut-out late on when he parried a powerful Negredo volley as the hosts belatedly pushed bodies forward.
The majority of chances arrived at the other end.
Alonso almost doubled the visitors' advantage within seconds of kick-off in the second half, the Spaniard forcing a fine low save from Valdes after Hazard helped on Victor Moses's cross.
The second goal failed to materialise, but Chelsea closed out their latest victory with relative ease to secure another valuable three points towards their growing title challenge.