Chelsea staged a stunning late comeback to snatch victory and wreck Mikel Arteta's return to Emirates Stadium as Arsenal manager.
Arteta, taking charge of his first home game since succeeding sacked Unai Emery, looked on course for three important points after Pierre-Emerick Aubameyang's early header put the Gunners in front.
Chelsea, with manager Frank Lampard making a first-half tactical switch when he introduced Jorginho for the struggling Emerson, were gifted an equaliser seven minutes from time when goalkeeper Bernd Leno hopelessly misjudged Mason Mount's free-kick to leave the substitute with a simple finish.
Arsenal were doubly frustrated because Jorginho was fortunate to escape a second yellow card for a cynical foul on Lucas Torreira - but Arteta's home bow was to get even more painful.
Chelsea struck again with three minutes of normal time left, Tammy Abraham turning Arsenal substitute Shkodran Mustafi far too easily in the area to beat Leno and complete the smash-and-grab raid.
Arsenal, who lost defender Calum Chambers to injury in the first half, will start 2020 11 points behind fourth-placed Chelsea and without a Premier League home win since 6 October.
It is the first time they have lost four consecutive home matches in all competitions since December 1959.