Gareth McAuley's late goal earned West Bromwich Albion a third straight league win to pile more pressure on Chelsea manager Andre Villas-Boas.
McAuley diverted in Liam Ridgewell's miscued shot to secure the points from an open match that raises further questions about Villas-Boas's future.
Daniel Sturridge missed two fine chances for Chelsea, who have only won three of their last 12 league games.
But the Baggies dominated the latter stages and were good value for the win.
Even the most ardent Blues fan would struggle to argue against a result which leaves them fifth in the table, three points behind an improving Arsenal side, and in serious danger of failing to qualify for the Champions League.
And the relentless pressure on the Portuguese will surely only increase if they fail to overturn a 3-1 first-leg deficit against Napoli in their Champions League last 16 tie on Wednesday, 14 March to progress in Europe.
Before they look to progress against the odds in Europe, Chelsea also face a tricky FA Cup fifth-round replay against Birmingham on Tuesday .
Owner Roman Abramovich has so far refused to publicy back the man he appointed at great expense in the summer and there was precious little Villas-Boas could point to from their display at the Hawthorns to convince Abramovich he deserves his unwavering support.
The Baggies were full of energy, won the midfield battle and denied Chelsea time and space to end a run of 11 straight Premier League defeats against the Blues.
Chelsea will point to two glorious opportunities spurned by Sturridge either side of the break.
And although the hosts had the greater share of possession and territory throughout, the Blues had the better chances before half-time.
Sturridge, back in the side despite limping out of the midweek England friendly against Holland with a toe injury, missed the best one when he dragged his shot wide after running on to a brilliant pass from Didier Drogba.
Gary Cahill also had a fierce 12-yard strike blocked b
