Arsenal has missed out on the chance to regain top spot in the Premier League after losing 1-0 at home to London rivals Chelsea.
Meanwhile, Swansea gave new coach Francesco Guidolin victory in his first game in charge with a 2-1 win away to Everton as the Welsh club moved four points clear of the relegation zone.
In London, Arsenal was reduced to 10 men in the 18th minute when Per Mertesacker was sent off for a challenge on Chelsea striker Diego Costa.
Four minutes later, Costa gave Chelsea the lead when he got on the end of a Branislav Ivanovic cross at the near post.
The defeat left the Gunners in third spot, three points behind shock leaders Leicester City, while Chelsea moved up into 13th place.
For all its contrasting fortunes this season, Chelsea is now unbeaten in nine league games against Arsenal.