Chelsea became the latest leading Premier League side to see its ambitions stall at West Bromwich Albion as a 1-1 draw prevented the leaders from bolstering their position on Tuesday.
Just like Everton and Liverpool in the previous two games at the Hawthorns, Chelsea was forced to settle for a goal apiece.
Chelsea defenders exploited slack opposition defending to put the visitors in front, with Branislav Ivanovic turning the ball into the net on the stroke of half time after David Luiz flicked on William's corner.
But Chelsea's defence left Victor Anichebe in too much space to meet Saido Berahino's cross in the 87th minute and head West Brom level.
"In the first half we had complete control without creating many chances — Petr (Cech) not making one save, their strikers with not one shot," Chelsea manager Jose Mourinho said. "In the second half they opened and gave us the space we didn't have in the first half, but we were not strong enough to kill the game."
Chelsea is two points clear of second-place Arsenal, which hosts struggling Manchester United on Wednesday. Manchester City also has a chance to move to the top by beating Sunderland if Arsenal slips up.
"We must have the personality to get out, to not accept the pressure," Mourinho said.
From the top to the bottom, the Premier League has rarely been so open in the second half of the season.
West Ham beat Norwich with a third consecutive 2-0 win to complete its rise from 18th to 10th in eight days, but the London club is still only eight points from bottom-place Fulham.
Mohamed Diame's cross was headed in by James Collins in the 84th and the striker netted himself in stoppage time after surging through on the break. Norwich is just 16th despite being only three points behind West Ham.
Cardiff remains mired in the relegation zone after being held to a 0-0 draw by Aston Villa.
At the other end of the standings, Southampton extended its unbeaten run to six matches after Jose Fonte clinched a 1-0 victory at Hull.
An initial header by Fonte was saved, Jay Rodriguez's follow-up was thwarted but Fonte had another attempt that was cleared by Maynor Figueroa — only for goal-line technology to show the ball had crossed the line. Southampton would have gone in front regardless, as Rickie Lambert fired the rebound over the line.