Chelsea missed out on the chance to extend their lead at the top after Victor Anichebe's late equaliser denied them victory at The Hawthorns.
The Blues looked like returning to London with three points in the bag when Branislav Ivanovic fired them ahead on the stroke of half time.
Chelsea missed chances to wrap up the win, with Samuel Eto'o and Willian denied by West Brom keeper Ben Foster.
And they were made to pay late on when Anichebe headed home Saido Berahino's cross to rescue a point for the Baggies.
Blues boss Jose Mourinho rued his side's lack of killer instinct as they dropped vital points in the title race.
He said: “My thoughts are for 60 minutes – only one team, and that team couldn't kill the game. Especially in the first 15 minutes of the second half.
“For 10 minutes the game was poor, and for the last 20 minutes, only one team – and that team fought a lot to get the goal and the point.
“In the end, I think they deserved it, because of their effort.”
