Arsenal rang in the new year by giving boss Mikel Arteta his first victory as lacklustre Manchester United crashed to a 2-0 defeat, while Tottenham and Chelsea also slipped up in the Premier League's top four race on Wednesday.
Arteta had watched Arsenal take only one point from his first two matches, with last weekend's 2-1 defeat against Chelsea especially painful after the Blues scored twice in the final minutes.
But Arsenal had played well for long periods in that London derby and they produced an even more sparkling display to put United to the sword at the Emirates Stadium.
The Gunners took the lead in the eighth minute when Sead Kolasinac's cross ran through to Nicolas Pepe at the far post and the Ivory Coast winger beat David De Gea from close-range.
Arsenal, who last won in the league at West Ham on December 9, got a deserved second goal in the 43rd minute when De Gea kept out Alexandre Lacazette's flick and Sokratis reacted quickest to lash home.
Arteta's team are nine points adrift of the top four, but their second win in 16 matches in all competitions offers hope of a brighter future under the former Arsenal midfielder's management.
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"It feels great, incredible. I'm so happy and proud of the players," Arteta said.
"Physically the level dropped in the second half but the commitment and desire was terrific.
"The performances are getting better so I was desperate for the win and we got it."
"We weren't ruthless enough. We got the goal and we had the possession. But we just didn't kill the game off."
- Spurs unravel -
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"I was rude," Mourinho said. "I clearly deserved the yellow card. I had bad words."
Elsewhere at the top of the table, second-placed Leicester cruised to a 3-0 win at