After a dream weekend where everything they wanted went their way, Chelsea stumbled at the next hurdle, with West Brom earning a 1-1 draw at the Hawthorns against the English Premier League leaders.
Chelsea could have gone four points clear of Arsenal with a victory at West Brom, but Victor Anichebe, who recently denied Liverpool all three points, got in on the act again, netting an equaliser with just three minutes remaining, after Branislav Ivanovic had given the Blues the lead late in the first half.
West Brom, who had found themselves consigned to the bottom three after results at the weekend, kept their plan simple, of weathering the Chelsea attacking storm - while remaining true to Pepe Mel's principles of putting pressure on the man with the ball -- while trying to get anything they can out of counter-attacks.
However, Chelsea, after pressing and probing for much of the first 45, broke through in the third minute of first half stoppage time, with Ivanovic slotting home from close range after David Luiz had touched the ball to his teammate following a corner from how-good-a-signing-is-he-proving-to-be Willian.
Chelsea had a couple of chances to put the game to bed early in the second half, but Albion held on before growing into the game themselves, with James Morrison, Thievy and Chris Brunt coming close.
I-score-goals-against-big-teams-now Anichebe was thrown in for the final 17 minutes or so, and he proved to be the difference, getting ahead of Luiz to turn in a cross from Saido Berahino for a share of the spoils.
"For 60 minutes there was only one team in the game but that team couldn't kill the game, especially in the first 15 minutes of the second half," Chelsea manager Jose Mourinho said.
"For 10 minutes the game was poor and for the last 20 minutes there was only one team in the game, and that team fought a lot to get the goal and the point. In the end they deserved it because of their effort. Their goal gives some truth to their reaction.
"We were not strong enough to kill the game. They had a reaction and we couldn't cope with the reaction. The team started being too deep, I was feeling that even if I put some attacking players it wouldn't help because the team in that moment was under pressure.
"It looked like it was impossible for my defenders to get out of the first third of the pitch. They fought for it and they deserved it."
The result means Chelsea are two points ahead of Arsenal, with Manchester City a further point adrift. The Gunners host Manchester United on Wednesday night, while City entertain Sunderland.
West Brom pulled ahead of the relegation zone, above Sunderland on goal difference, and Mel said he was proud of his side's performance against the rampant Chelsea.
"For me it was a pleasure because the players showed good work and now, in the changing room, it is calmer and there is a good sensation," Mel said after the draw on Tuesday. "Chelsea are a top team in Europe and West Brom in the second half were better. Now I am a proud coach.
"Today was good for me. There were two different halves. The first half was good defence and only two chances for Chelsea. And in the second half there was good transition from defence to counter-attack.
"We showed a good mentality. If West Brom scored a goal earlier than we got the equaliser I'm sure we would have won the match. It is the best performance since I've been here. Chelsea is a good team, they are top. The players showed a good mentality to win."
Other results: Cardiff 0-0 Aston Villa; Hull 0-1 Southampton (Jose Fonte 69th); West Ham (James Collins 84th, Mohamed Diame (90+4) 2-0 Norwich.