Jose Mourinho claimed that his Chelsea team was too relaxed as they allowed West Brom to snatch a point just three minutes from time.
The Blues went into the game as Premier League leaders and looked set to seal a 1-0 win over the Baggies, but Victor Anichebe grabbed a late equaliser on a night when Chelsea barely looked like potential champions.
They had been poised to move four points clear at the top of the table on the back of defender Branislav Ivanovic's injury-time strike in the first half at The Hawthorns, but substitute Anichebe flicked home his third goal of the season in the 87th minute to make it 1-1 and thwart the Blues, who had lost on their last two visits to this part of the West Midlands.
"We were comfortable in the game, maybe too comfortable. We didn't kill the game when we had the chance," said Mourinho.
"I was feeling that the only thing we could do is defend well. We were not strong enough to get out in the last 10 minutes.
"The game doesn't end until the last whistle, and the defensive mistake led to a goal."
Mourinho was positive, however, as Chelsea moved a further point ahead of Arsenal and Manchester City at the top, but both could now go above the Blues if they win their games tonight.
"In this moment we have one more point than before," Mourinho explained. "If Arsenal and City win tomorrow, they go above us, but that is their job.
"Every game is difficult, and every team needs points."
After all the hype in the wake of their stunning win at Manchester City and Eden Hazard-inspired thumping of Newcastle on Saturday, Mourinho's side were off the boil against the Baggies.
Only Willian can claim to have had an impressive game as it was he who primarily pulled the strings and who crucially set up the only goal of the first half just as Albion were on the brink of heading into the interval on the back of a creditable 0-0 scoreline.
From Willian's corner into the near post, David Luiz was left unmarked to flick the ball on with his right foot to the far post where Ivanovic slotted home from three yards.
At least the home side showed greater attacking intent in the second period as James Morrison fired wide, Thievy Bifouma flashed a shot through the six-yard box, while skipper Chris Brunt hammered a half -volley beyond the left-hand post.
With Anichebe on for Bifouma in the 73rd minute, the Nigerian glanced home Saido Berahino's cross three minutes from time to lift Albion back out of the relegation zone.
It was a fair result and all credit to Albion who pressed and harried admirably throughout the entire game, even if they failed to record a single shot on target in the first 45 minutes.
It has been the desire of new head coach Pepe Mel to see his players give the opposition not a minute's respite, and Chelsea were rarely allowed any time on the ball.
That was underlined by the minimal number of chances that came their way during the opening half prior to Ivanovic's injury-time effort.
The key ones fell Willian's way as he first saw a fierce 27th-minute drive flash just over the angle of bar and post.
Three minutes from the break he picked up a loose ball on the edge of the area before chesting it down and sending a half-volley dipping over the goal.
Then came Ivanovic's goal – a blow for Albion, although they had conjured just one chance of note in the first period.
The first half was most notable for the boos that rang in the ears of Ramires – playing in place of Frank Lampard in the only change to the Chelsea team that beat Newcastle United.
Ramires was the player who contentiously earned Chelsea a penalty in the 2-2 draw at Stamford Bridge in October, when Albion were seconds away from a shock victory.
The Albion fans certainly had not forgotten that moment as Ramires' every touch was greeted with jeers and catcalls.
After the break Willian came the closest to extending Chelsea's lead, but found Foster in fine form.
As the half wore on the home side finally found some impetus up front and began to create their opportunities prior to Anichebe coming up trumps to send the home fans into raptures.