Chelsea edged rivals Arsenal 1-0 at the Emirates Stadium courtesy of Diego Costa's first-half goal, while Per Mertesacker was dismissed.

Sportsmail's MATT BARLOW picks out five things that we learned from the Premier League contest.

Diego Costa scored the decisive goal as Chelsea beat 10-man Arsenal at the Emirates Stadium on Sunday

1. Arsenal’s midfield are starting to feel the absence of injured midfielders Francis Coquelin and Santi Cazorla. Chelsea played on this soft-spot, with considerable success in the first-half.

There was a recall for Nemanja Matic to supply extra legs, while leaving John Obi Mikel to stifle Mesut Ozil.

It worked brilliantly in the opening phase, when Matic, Willian and Diego Costa hustled Arsenal into errors, deep in their own territory and allowed Cesc Fabregas and Oscar, drifting inside from the left, the room to operate.

Arsene Wenger must hope Mohamed Elneny, signed from Basle, soon adjusts to fill this void, because others will notice how Aaron Ramsey and Mathieu Flamini were over-run, even before they were reduced to 10 men.

Nemanja Matic takes on Aaron Ramsey in one of the game's fascinating midfield battles

Willian (pictured) and Matic worked tirelessly, forcing Arsenal into errors that allowed other to capitalise on

2. Branislav Ivanovic is back to his best. Or somewhere near it. He supplied a splendid cross for Diego Costa’s goal and had a header cleared off the line from a corner.

No surprise Chelsea dominating in the air from set-pieces after Arsenal lost Mertesacker and Giroud. But, moreover, Ivanovic played with that old chest-out swagger and the manner of a runaway train.

He was unruffled by the threat of Theo Walcott and Nacho Monreal, who have combined well on the Arsenal left in recent weeks. Moreover he typified the winning mentality which has long been associated with Chelsea.

It went missing earlier in the campaign, and they have been vulnerable at the back, even under Guus Hiddink, but there is plenty of experience in the defensive unit.

When John Terry and Ivanovic are in the right mood, they know how to defend a lead. “That’s why we’re champions” as the Chelsea supporters put it.

Branislav Ivanovic (left) was excellent and delivered the cross that led to Diego Costa's opening goal

Ivanovic carries the ball away from Alexis Sanchez during the showdown at the Emirates Stadium

Ivanovic challenges Arsenal dangerman Mesut Ozil as Arsenal try to go on the attack

3. The cauldron of anxiety that is the Emirates Stadium does not help Arsenal. Everyone agrees this is their best chance in years of winning the title, and the nerves set in early when things started to stack up against them.

Arsene Wenger was jeered for replacing Olivier Giroud in the first-half in a reshuffle to cope with the red card for Per Mertesacker. Presumably these were not the Arsenal fans who often deride Giroud for missing chances.

Walcott was jeered for losing possession and Mathieu Flamini for failing to convert a kung-fu volley before half-time.

Sixty thousand were delighted to see Alexis Sanchez back and the decision to replace Walcott with Alex Oxlade-Chamberlain provoked ironic cheers.

Even in an era of audience feedback and interactivity, it helps to get behind the team, especially as they venture into the business end of the season.

Arsene Wenger's decision were jeered by the Emirates crowd, especially when Olivier Giroud was taken off

Giroud looked unhappy to be sacrificed after the sending off of Per Mertesacker in the first-half

4. Hiddink does not want to put out the fire in Diego Costa’s belly. And you can see why. On this knife-edge, he will win games.

Costa emerged in ferocious mood, determined to torment Mertesacker and Laurent Koscielny with his full repertoire of annoying habits, right down to leaving the pitch at a snail’s pace when he was substituted.

He pulled left to play on Mertesacker and crashed to the ground when the German was foolish enough to slide another ball he was never going to reach.

Manchester City’s Martin Demichelis escaped a very similar foul with a yellow card yesterday, which seemed lenient at the time, and Mark Clattenburg went for a red one.

Costa bought this one. Perhaps it was a sense of revenge after the retrospective three-match ban he received after the clash at Stamford Bridge in September. Gabriel Paulista had been on for less than a minute when Costa scored, proving Hiddink’s point neatly.

Diego Costa was in typically combative mood at the Emirates and scored the decisive goal

The Chelsea striker remonstrates with referee Mark Clattenburg during the contest at the Emirates Stadium

5. The return of Sanchez offers Arsenal hope. The Chilean lifted spirits when he came on for the last half-hour or so, looking fresh and powerful.

He helped the home team summon a strong finish despite their numerical disadvantage. And his presence will be vital as Wenger’s team attempt to respond to this disappointment.

They are only three points off the top, still very much in it and do not have to play Chelsea again. It is now more than three years since they scored against Chelsea in the Barclays Premier League. Arsenal’s mental block has survived Jose Mourinho.

The return of Alexis Sanchez will strengthen the chances of Arsene Wenger's men winning the title