Arsenal 0-1 Chelsea
Arsenal missed out on the chance of going back to the top of the Premier League table with a damaging home defeat to Chelsea.
The Gunners were reduced to ten men early at the Emirates Stadium when Per Mertesacker pulled down Diego Costa to deny the Blues’ striker a clear goal-scoring chance.
And it was Costa who popped up to score what was to prove the winning goal five minutes later.
The result now leaves Arsenal in third place, level on points with Manchester City and three behind leaders Leicester City.
It was a good weekend for the Foxes, the surprises of the season so far, as Tottenham were the only other top six side to win at the weekend.
Once again Costa was the villain of the piece from an Arsenal point of view.
The Spain international tangled with Gabriel in the champions’ 2-0 victory earlier in the campaign and saw the Arsenal defender sent off.
It was German World Cup winner Mertesacker who saw red this time for bringing down the controversial striker on 18 minutes.
Gabriel came off the bench in place of Olivier Giroud but Costa soon had the ball in the net for the game's only goal as Chelsea moved further away from trouble at the bottom end of the table and put a dent into Arsene Wenger's title bid.
Mertesacker became the fourth Arsenal man sent off in their last five meetings with Chelsea and the Gunners have now failed to beat their London rivals in the league since 2011.
Remarkably they have not even scored against the Blues in a Premier League meeting for more than three years.
Oscar had the game's first decent chance after being gifted possession by a nervy Laurent Koscielny and his low shot earned the visitors a corner as former Chelsea goalkeeper Petr Cech was forced to push the ball behind.
Arsenal were looking to break into the Blues box but Chelsea gained possession and broke forward at pace, with a defence-splitting ball from Willian sending Costa away.
The forward, such a thorn in the side of the Gunners backline, was then sent sprawling by the back-tracking Mertesacker with referee Mark Clattenburg, above, showing the expected red card.
Costa then made things worse as he popped in between Koscielny and Gabriel to prod home Branislav Ivanovic's tempting cross.
There was frustration for Arsenal on the stroke of half-time as Mathieu Flamini turned Aaron Ramsey's delivery over the crossbar despite being free in front of goal.
After the break Wenger sprung Alexis Sanchez from the bench as the Chilean made his comeback after two months out with a hamstring problem.
Arsenal committed more and more men forward and Ramsey nearly benefited when Thibaut Courtois failed to claim a cross.
However, Chelsea saw out the remaining minutes to claim three points which move them up to 13th in the table and leave Arsenal's title hopes back in the balance.
