Mata (10) had put Chelsea ahead in the first half
Rafael Benitez remains without a win as Chelsea manager after West Ham came from behind to beat struggling Chelsea.
Juan Mata had put the Blues ahead with an assured side-footed strike, reports the BBC.
Carlton Cole nodded in to level, although the visitors claimed the striker fouled Branislav Ivanovic, before a Mata free-kick hit the post.
Chelsea survived when Winston Reid's header was cleared off the goalline but strikes from Mohamed Diame and Modibo Maiga completed the fightback.