Chelsea's Costa is again a thorn in Arsenal's side

By Jamie McGeever LONDON (Reuters) - Controversial Chelsea striker Diego Costa proved to be Arsenal's nemesis once again on Sunday, scoring the only goal of a fiery London derby to deny the home side the chance to top the Premier League. The Spaniard also drew the foul from Arsenal defender Per Mertesacker early in the first half which resulted in a red card for the German, a setback from which the Gunners were unable to recover despite the return from injury of their Chilean striker Alexis Sanc