Victor Anichebe struck in the 87th minute as West Bromwich Albion prevented Chelsea from opening up a four-point lead in the Premier League in a 1-1 draw on Tuesday.
Chelsea were bidding to pull clear of the chasing pack and Serbian defender Branislav Ivanovic put them on course to do that when he slammed home from David Luiz's back-heel in first-half injury time.
Oscar, Samuel Eto'o and Willian all spurned chances to extend the visitors' lead and West Brom forward Thievy Bifouma almost punished them with a low shot that flashed across the face of goal in the latter stages.
Jose Mourinho's side appeared to have escaped, but with three minutes remaining Saido Berahino curled in a cross from the left and Anichebe got in front of Luiz and goalkeeper Petr Cech to head home the equaliser.
The Nigerian striker's goal means that Chelsea could slip to third place on Wednesday, having only taken over at the summit at the weekend.
Second-place Arsenal will return to the top if they win at home to Manchester United, while third-place Manchester City can leapfrog Chelsea on goal difference by overcoming Sunderland.
"We were comfortable in the game, maybe too comfortable. We didn't kill the game when we had the chance," said Chelsea manager Mourinho.
"In this moment we have one more point than before. If Arsenal and City win tomorrow (Wednesday) they go above us, but that is their job. Every game is difficult and every team needs points."
West Brom, meanwhile, clambered out of the relegation zone at Sunderland's expense.
"I am very pleased. The players worked hard and were strong. If we scored the goal earlier, we would win the game," said West Brom manager Pepe Mel.
"1-1 against Everton, Liverpool and Chelsea. Now we need to win our next game (at home to Fulham on February 22). West Brom next season will be in the Premier League."
Second-bottom Cardiff City squandered an opportunity to haul themselves towards safety after being held to a 0-0 draw by Aston Villa.
Cardiff hit the woodwork twice in the space of a minute during the first half, with Fraizer Campbell and Craig Noone the players denied, in a game that left Ole Gunnar Solskjaer's two points below West Brom.
West Ham United eased their own relegation fears with a 2-0 win at home to struggling Norwich City.
The visitors created chances throughout the game, but West Ham stole ahead in the 84th minute when centre-back James Collins nodded Mohamed Diame's left-wing cross past Norwich goalkeeper John Ruddy.
Diame struck with a deflected shot in the fourth minute of injury time to complete a third consecutive victory without conceding a goal for Sam Allardyce's West Ham, who climbed to 10th place in the table.
"I'm delighted. It was a hell of a game Norwich gave us tonight," said West Ham manager Allardyce.
"Our goalkeeper kept us in the game and we scored late, when it is hard for anybody to come back into it. It's a great end to an important run of fixtures."
In the evening's other match, Southampton leapfrogged Newcastle United into eighth place, and closed to within two points of defending champions Manchester City, with a 1-0 success at Hull City.
Portuguese defender Jose Fonte settled the game in the 69th minute at the KC Stadium, ramming home at the third attempt after his header was saved by Steve Harper and Jay Rodriguez's follow-up was blocked.