This crazy, roller-coaster Premier League title race took another twist late on here when West Brom sub Victor Anichebe equalised to thwart Chelsea's hopes of going four points clear.
Anichebe's header, cancelling out Branislav Ivanovic's first-half strike, was reward for the hosts' efforts and hugely enterprising tactics of Pepe Mel in the second period, playing with three forwards much to the delight of West Brom fans and the ultimate frustration of Jose Mourinho.
West Brom suffered an injury to Claudio Yacob, but Mel used the situation to send on Youssouf Mulumbu, who helped the hosts claim control of midfield.
Mourinho will also surely rue not taking off Gary Cahill when the centre-half was hobbling and then Anichebe pounced.
Before Ivanovic swooped just before half-time, scoring his second goal in three games, there had been little football of sufficient quality to warm the crowd. West Brom counter-attacked occasionally, while Chelsea enjoyed plenty of possession, and Eden Hazard flitted about busily, but did not threaten Ben Foster's goal until injury-time.
Ivanovic's goal changed all that, meaning that West Brom had to open up more.
The sides have fallen out this season, ensuring some tensions on the pitch.
West Brom believed that Mourinho accused them of being a "Mickey Mouse" club during the fractious encounter at the Bridge, a claim Chelsea contest.
Until injury-time, there was little of note. Samuel Eto'o and Hazard linked up with occasional promise. Eto'o beat Craig Dawson, cutting the ball back to the sliding Ivanovic but Yacob had dropped back to clear the danger.
Possession was predominantly in Chelsea's sway. Nemanja Matic sat in front of the back four, occasionally muscling his way upfield.
Willian was drifting around, contributing to some intricate moves, trying to force the breakthrough with a shot over.
West Brom, lining up 4-3-3, lacked an obvious target man with Shane Long sold.
Saido Berahino was interchanging with Thievy Bifouma, a mix of pace and unpredictability, in the centre. Morgan Amalfitano brought some neat touches and energy to the right.
As the half closed, Chelsea stepped up the pace. Ramires had a shot deflected wide after a good build-up between Oscar and Hazard and Chelsea's elegant Belgian had a free-kick blocked. Willian shot over.
Then West Brom's marking let them down badly. Willian swept a corner across from the right, David Luiz flicked on and there was the unmarked Ivanovic striking the ball home from six yards.
Chelsea seemed largely in control. Oscar beat Liam Ridgewell easily for pace. Willian moved away from Yacob and sent in a powerful shot pushed away by Foster.
Chelsea looked the likelier to score, Hazard's pass teed up Eto'o, whose shot piled into the side netting. That was the end of a hard-working shift from Eto'o, who was replaced by Fernando Torres with 21 minutes remaining.
Torres was immediately involved in a move with Hazard and Oscar that culminated in Willian's shot thudding into Foster's gloves.
Mel had already replaced the limping Yacob with Mulumbu, who immediately brought more order to West Brom's order, and then sent on Victor Anichebe for Amalfitano, going for all-out attack. Thievy fired wide. James Morrison went close.
Ivanovic then boiled over, screaming at the assistant ref, and being cautioned by Anthony Taylor. Petr Cech had to intervene to calm Ivanovic down.
Mel made his final move, withdrawing Thievy for Matej Vydra. Mourinho looked to close up the game, taking off Oscar for John Obi Mikel.
Luiz almost conceded a penalty when pulling Anichebe's shirt and West Brom frustration built as they felt Chelsea were engaging in time-wasting.Cahill sustained an injury and stayed down before hobbling around the apron of the pitch, before coming back on. Chelsea's defence seemed disorganised and when Berahino whipped the ball in with three minutes remaining, Anichebe was there to flick in a header off the post for a dramatic leveller.
Mourinho changed his replacement plans, sending on Mohamed Salah instead of Frank Lampard for Ramires. Assistant referee David Coote then signalled six minutes of added time and Chelsea earned a lucky corner just before the end when Foster had actually kept the ball in. The 'keeper then did brilliantly to clutch the ball and avert the danger.
Taylor blew the whistle for full time, leaving Chelsea frustrated but West Brom deserved a point for their prodigious effort in the second half. (© Daily Telegraph, London)