West Bromwich Albion 1-0 Chelsea
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 by the on-rushing James Morrison, and the lively Juan Mata tested keeper Ben Foster from close range after great build-up play involving Michael Essien and Drogba.
But it was Sturridge's second opportunity after the interval that will play heavily on his mind. The striker tried to let the ball run on as Foster came tearing out of his penalty area and the keeper read Sturridge's mind and was able to clear.
Lampard also went close with a low strike in injury-time.
However Albion bossed the final 30 minutes and Petr Cech, whose only notable first-half save came from Morrison's long-range effort, was forced into a number of good stops.
The Czech number one made a superb low block from Marc Antoine-Fortune and tipped over a fierce Youssouf Mulumbu strike.
And although many of Cech's other saves were fairly routine, the home side's pressure finally told against an increasingly ragged Chelsea side.
The Blues were desperately piling forward in search of a winner but the Baggies retained their organisation and composure and secured a deserved victory with eight minutes remaining.
Chelsea failed to clear a corner and McAuley turned the ball in from close range from Ridgewell's scuffed shot following a Keith Andrews cross.
Albion held on with relative ease to move 13 points clear of the relegation zone.