LONDON — Long resigned to surrendering the trophy, Chelsea made its impact on the top of the table Sunday by inflicting 10-man Arsenal's first home loss since the opening day of the Premier League season.
The 1-0 defeat saw Arsenal slip from first to third this weekend and allowed Leicester to regain a lead which was as unanticipated in August as Chelsea's freefall.
It was yet another uncomfortable day for Arsenal manager Arsene Wenger against Chelsea, which hasn't lost to its London rival in the league for five years.
Wenger was booed by his own fans over a bewildering substitution straight after Mertesacker's sliding tackle on Diego Costa.
Gabriel was brought on to replace forward Olivier Giroud and the defender failed to track Costa at the near post in the 23rd minute when Chelsea went in front.
"You want to make a poll for every decision to see who does what? I made the decision and for me it was quite normal," Wenger said in response to questions about the substitution. "We had to go for long distances and we needed pace to go from one goal to the other. We knew we had to drop deeper and use pace to get forward."
Wenger blamed Costa for getting his defender sent off.
"Mertesacker made the challenge but I said he got Mertesacker sent off, you cannot say it's not true," Wenger claimed.
Arsenal never recovered from the early double blow, falling three points behind leader Leicester and giving Chelsea one of the most comfortable days of a pitiful title defense.
The result extended Guus Hiddink's unbeaten run since replacing fired Jose Mourinho to eight games, although the team has still only moved up to 13th place. The fourth Champions League place, 14 points away, looks unobtainable.
"I'm very pleased with the attitude of the team," Hiddink said. "I'm also pleased with the players, the way the team tried to play. It was not just reactive but proactive.
"We showed ambition going forward ... the only thing you can criticize a bit — we didn't go for the kill."
Arsenal has now collected only two points from its last three games as it chases a first league title since 2004.
The Gunners rarely looked like scoring. Mathieu Flamini flicked the ball over with the final touch of the first half while unmarked in front of goal — a display of extravagance when precision was required.
Even Alexis Sanchez's return in the second half, after two months out injured, made little impact. In a goalmouth scramble when the ball pinged between Arsenal players, none were able to apply the finishing touch, with Sanchez managing an air-shot.
It summed up Arsenal's derby day.
But for Chelsea it was a day when Cesc Fabregas, controlling midfield against his former club, and Costa thrived. It's a far cry from the miserable days under Mourinho earlier in the season.
"I try to focus them on what they are good at and not lose energy on other things," Hiddink said. "I think it's good what they are showing."