West Brom boss Tony Pulis hailed Arsenal's players after their shock win.
James Morrison's volley and a Mikel Arteta own goal gave the hosts - who only managed one shot on target - victory after Olivier Giroud opened the scoring.
Pulis said: "Once we got 2-1 in front we knew Arsenal would throw everything at us and they did in the second half. The lads were very resolute and determined. I'm really pleased for them.
"Arsenal are a top top team. I thought Arsenal were very good, particularly second half when they ramped it up.
“Alexis Sanchez was just wonderful - his movement, his play.
“You've also got Mesut Ozil and Santi Cazorla who have wonderful talent.
“It's a great result for us and we're very very pleased."
Source: tribalfootball.com