Emerson Royal received a straight red card as Arsenal beat Tottenham at the Emirates Stadium.
The north London derby is always a feisty encounter and Saturday's 12:30 kick-off turned out no different.
Arsenal took the lead in the game courtesy of a Thomas Partey screamer, a goal which bellowed yet more life into an already intense affair.
But Tottenham responded just over 10 minutes later, when Harry Kane converted from the penalty spot and drew the scores level going into half-time.
Both teams came out fighting after the break, but it was Arsenal who carried over their strong momentum and seized the advantage in the game via a 49th minute Gabriel Jesus tap in.
The away team needed a response but a careless moment from right back Royal looks to have cost them dearly.
The Brazilian clumsily took out Arsenal's Martinelli with a shin high tackle on the back of his leg.
It really was a moment of madness from Royal. Check it out below.
WATCH: Royal's red card challenge
What on earth was Royal thinking there? The referee had no choice but to show the Spurs man a red card, leaving his team fighting short-handed for the rest of the derby.
Unsurprisingly, it took the Gunners just five minutes to capitalise on Spurs' misfortune when Granit Xhaka made it 3-1 in the 67th minute.
It was a beautifully worked goal with Martinelli, who had been excellent all game, laying it into the Swiss international's path, who duly converted.
WATCH: Martinelli and Xhaka combine for the third
Exceptional stuff from Mikel Arteta's men.
The victory means the Gunners remain top of the Premier League table and now move four points clear of their closest competitor Manchester City.
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Although they face a huge test next week as they welcome Jurgen Klopp's Liverpool side to the Emirates in Sunday's 16:30 kick off.
The Reds are currently languishing in eighth place in the table and can't afford to give up any more ground to this red-hot Arsenal side.