West Brom striker Peter Odemwingie made it a miserable return to the Hawthorns for Chelsea manager Roberto Di Matteo with a second-half winner to stun the Champions League holders.
The Nigerian international headed home a cross from Shane Long to secure a 2-1 victory.
It condemned Di Matteo, who spent two years as Albion boss, and his side to a first away Barclays Premier League defeat of the campaign.
But his former club, managed by ex-Chelsea number two Steve Clarke, are dreaming of Europe after registering a sixth win in seven home games.
The Blues dominated for long periods territorially with Victor Moses and Daniel Sturridge causing problems.
But they found keeper Boaz Myhill, who was deputising for the injured Ben Foster, in inspired form with several crucial saves.
Chelsea clearly missed the physical presence at the back of injured skipper John Terry who will be side-lined for three weeks with a knee injury.
Long and Odemwingie fully capitalised on the absence of the former England defender as they punished some slack defending.
Long appeared on a one-man mission to honour his grandmother who died yesterday with another superb performance.
The Republic of Ireland international revealed a T-shirt which said 'RIP Nan' after scoring the opening goal.
Chelsea dominated possession in the early stages and Ryan Bertrand came close to putting them ahead after eight minutes.
Moses made a powerful run to the byline and squared the ball back across the danger area to Bertrand whose low left-footed drive was cleared off the line by Liam Ridgewell.
But 60 seconds later the Baggies took the lead via their first worthwhile attack.
Zoltan Gera played the ball back to James Morrison whose floated cross picked out Long who got in behind David Luiz to head home from close range.
It was the Republic of Ireland international's sixth goal of the campaign.
Chelsea tried to retaliate and John Obi Mikel's low 30-yard drive was only just off target with Myhill scrambling across his line.
Myhill then got his body behind a swerving first time attempt from Sturridge.
Albion were at their most threatening on the counter-attack and Long, under pressure from Gary Cahill, had a shot saved by Petr Cech after attacking a cross from Billy Jones.
Long was then just off target with a curling drive after he had shaken off the challenge of Cahill.
But after 38 minutes Eden Hazard levelled for the Blues with his fourth goal of the season.
He attacked Cesar Azpilicueta's deep cross and was able to squeeze the ball past Myhill at the far post with a powerful header.
Chelsea continued to dominate possession and Moses fired over after being teed up by Cahill.
But Odemwingie restored Albion's lead five minutes into the second half.
This time Long was the creator with a cross to the near post and the Nigerian international got in front of Bertrand to glance his header past Cech for his fourth goal of the campaign.
Di Matteo made a double change with the ineffective Fernando Torres and Oriol Romeu making way for Juan Mata and Oscar respectively.
Myhill made a fine save to turn aside Sturridge's close range flick after Mata's ball had opened up the West Brom defence.
He then went down to cling on to Hazard's deflected shot as Chelsea delivered wave after wave of attacks.
Sturridge should have made it 2-2 with three minutes left but screwed his shot across the face of goal.