Jose Mourinho’s team were far from the fluent and formidable form which has characterised their recent ascent to the summit, however once they led via Branislav Ivanovic’s goal in first-half added time, they had looked on course for another victory.
Yet substitute Victor Anichebe, on the pitch 14 minutes as a replacement for Morgan Amalfitano, had other ideas as he headed home in the 87th minute to secure a precious point which lifted the Baggies out of the bottom three.
It was just as significant a goal at the top too as Chelsea were forced to settle for a lead of two points, as opposed to four.
“We were not strong enough to kill the game. They had a reaction and we couldn’t cope,” said Jose Mourinho afterwards.
West Brom captain Chris Brunt was proud of his side’s display: “We grafted away in the second half. As the game went on there was only one team winning that.”
The result gives both Arsenal and Manchester City, both in action tonight, the chance to claim top spot.
West Brom, meanwhile, will take confidence from a third successive draw at home under Pepe Mel ahead of a meeting with fellow relegation battlers Fulham on Saturday, yet they have still won just once in their previous 16 league matches.
New arrival Thievy Bifouma, on loan from Espanyol, was handed a first Baggies start, yet the major piece of team news surrounded the inclusion of Ramires replacing Frank Lampard.
The Brazil international was roundly booed by the home crowd in what was the sides’ first meeting since he controversially won a penalty which earned the Blues an injury-time equaliser in November.
Despite their vastly contrasting recent form, it was a surprisingly even start to proceedings with Mel’s renowned pressing game serving to frustrate the visitors.
It took Mourinho’s team until the 22nd minute to threaten when Samuel Eto’o’s cutback from the byline almost found the onrushing Ivanovic, with a combination of Gareth McAuley and Claudio Yacob doing just enough to avert the danger.
Mel had previously claimed victory over Mourinho when his Real Betis team pulled off a shock 1-0 victory over Real Madrid in November 2012.
His tactics appeared to be working against the Portuguese again and, with space squeezed in the Blues’ final third, a Willian long-range effort which flashed past the right-angle was as close as they came in the first half.
The Baggies looked a side transformed to the one who toiled in the weekend defeat at Crystal Palace which saw them fall into the bottom three. Pivotal to that were rookie forward duo Thievy, 21, and Saido Berahino, 20, whose trickery and tireless chasing made them a handful.
The pair contributed to the Baggies’ best opportunity of the half when Berahino’s cross was laid off to Chris Brunt by Thievy, yet the Northern Ireland international dragged his shot wide.
Chelsea always looked the more likely to score and the home side survived an anxious moment when Gareth McAuley sliced a Blues cross over his own crossbar.
However, their resistance was finally broken deep into the two minutes of first half added time when Willian’s corner was flicked on by David Luiz and the unmarked Ivanovic swept home from close range.
West Brom were out early for the second half, yet Chelsea made the more purposeful start upon resumption with Eto’o’s fierce strike repelled by Ben Foster.
Having been branded the “best young player in the world” by Mourinho following his blistering weekend hat-trick against Newcastle, Eden Hazard found himself in the shadows.
Instead, Oscar was the pick of Chelsea’s three amigos and he extended Foster again with a powerful long-range effort.
Fernando Torres was introduced for Eto’o as Mourinho looked to finish off the game, yet Chelsea were given a scare late on when Thievy and Brunt both spurned openings.
And, just as it appeared the home side had run out of chances, substitute Anichebe struck with a fine glancing header which crept in off the right post from Berahino’s teasing cross.
It was the third successive match in which a Baggies substitute has scored and Anichebe was the hero once more after stepping off the bench to rescue a point against Liverpool.
WEST BROM: Foster, Reid, McAuley, Dawson, Ridgewell, Morrison, Yacob (Mulumbu 63), Brunt, Amalfitano (Anichebe 73), Thievy (Vydra 78), Berahino.
CHELSEA: Cech, Ivanovic, Cahill, Luiz, Azpilicueta, Ramires (Salah 89), Matic, Willian, Oscar (Mikel 79), Hazard, Eto’o (Torres 69).
Ref: Anthony Taylor.