Marko Arnautovic's first West Ham goal secured a 1-0 victory over Chelsea and a first win for manager David Moyes.
Arnautovic struck after only six minutes to condemn the champions to a first defeat in seven Premier League matches.
The £24million club-record signing was jeered by home fans earlier in the season after some below-par displays, but he looks a player reborn under Moyes.
In fact West Ham look an entirely different side to the shambles they appeared towards the end of Slaven Bilic's spell in charge.
Moyes showed he is not one to shy away from tough decisions, and dropping England goalkeeper Joe Hart in a World Cup year was certainly a big call.
But Hart's replacement, Spanish keeper Adrian, justified his selection with fine first-half saves from N'Golo Kante and Davide Zappacosta.
Chelsea were carved open far too easily with West Ham's first attack, when Arnautovic played a quick one-two with Manuel Lanzini.
The Austrian nipped in behind Andreas Christensen and, as Cesar Azpilicueta stood off, curled his shot around the defender and keeper Thibaut Courtois.
Chelsea had plenty of time to respond, yet this was a strangely subdued display from a side with six wins from their last seven outings.
Alvaro Morata, often isolated in attack, cut a frustrated figure and was lucky to escape a booking for a blatant dive as he ran past Winston Reid.