Pedro and Alvaro Morata gave Chelsea a 2-0 lead within 20 minutes as the Blues dominated Arsenal at Stamford Bridge but goals from Henrikh Mkhitaryan and Alex Iwobi ensured Arsenal drew level before half-time.
Marcos Alonso scored the winner in the second half after good work from Eden Hazard.
Chelsea boss Maurizio Sarri has revealed Chelsea threw the two-goal lead to Arsenal due tactical mistakes made by the players.
“I think we have done very well for 75 minutes,” Sarri said.
“15 minutes at the end of the first half was horrible. We lost distances, I think. We are not able to recover the lost ball, or press in the other half as a team. Only one player, two players, and the others too far [away].
“I think the players were very good in the second half because it was not easy to answer the final part of the first half, from a mental point of view.”