Pedro put the Blues ahead after nine minutes by sweeping in an Alonso cross on the counter, before Alvaro Morata doubled the lead with a smart finish following Cesar Azpilicueta’s long ball.
Pierre-Emerick Aubameyang spurned a glorious chance just moments before Morata’s strike, and Henrikh Mkhitaryan also blazed over from a fine position in the box after the Gunners went two goals behind.
But Unai Emery’s side continued to cut Chelsea open and they hauled themselves level in dramatic fashion, scoring two goals in four minutes as Mkhitaryan drove in fiercely from the edge of the box before Alex Iwobi converted the Armenian’s cut-back before the break.
After half-time, Ross Barkley brought an impressive save from Petr Cech and N’Golo Kante saw a promising shot blocked by Sokratis, while Aubameyang had a goal correctly ruled out for offside on the break.
Chelsea’s Kepa Arrizabalaga almost spilled a cross straight to substitute Alexandre Lacazette, and Kante might have done better with a close-range header in the box.
But Alonso made no mistake with a tidy finish from close range after substitute Eden Hazard’s run and pass across goal set up the winner.
