The Gunners took three points thanks to Eddie Nketiah's brace in the first 10 minutes of the game.
Ironically, Nketiah, 21, was part of the Chelsea Academy until the age of 16. he was let go because the coaches thought he was too small. Arsenal offered him a scholarship within two days after that.
His path to the first team was not straightforward, but he has become a regular starter recently. He has already faced Chelsea - and grabbed another brace in the 4-2 win at Stamford Bridge last month.
It is squarely a three-horse race for two places in the top four who will play in the Champions League next season, with Tottenham a few points behind.
Arsenal and Tottenham are yet to play each other, so Chelsea hold a narrow advantage over their London rivals.
