West Brom defender Gareth McAuley increased the pressure on Chelsea boss Andre Villas-Boas with a late winner in the 1-0 Barclays Premier League clash at the Hawthorns.
McAuley struck eight minutes from time with the only goal to earn Albion their first ever Premier League win over the Blues.
It leaves Chelsea three points adrift of Arsenal, winners at Liverpool earlier in the day, in the battle for a Champions League spot.
Daniel Sturridge seized onto a casual pass from Jonas Olsson and squared the ball but Juan Mata failed to make proper contact.
Liam Ridgewell came to Albion's rescue when he blocked a goalbound attempt from Gary Cahill.
Ben Foster then leapt acrobatically to turn aside a fierce Michael Essien effort and before snatching the loose ball with Didier Drogba in close attendance.
Sturridge should have broken the deadlock in the 40th minute after a superb pass from Drogba created the opening, but the England striker screwed his attempt wide.
Sturridge missed another great chance to put the Blues in front after 55 minutes. He ran onto a pass from Mata but lacked the belief and confidence to take the ball around Foster who had raced out of his area.
Petr Cech turned over a fierce drive from the impressive Youssouf Mulumbu and also reacted sharply to block a low shot on the turn from Marc-Antoine Fortune.
It needed a last ditch challenge from Ashley Cole to deny Chris Brunt, but after 82 minutes McAuley put Albion ahead. Ridgewell drove Keith Andrews' chip across the area and McAuley diverted the ball past Cech.