The gruelling schedule for the Premier League's top clubs is taking its toll but that is leaving the title race wide open ahead of Wednesday's top-of-the-table clash.
Leaders Tottenham Hotspur travel to second-placed champions Liverpool on Wednesday with both sides on 25 points.
But eight teams are within five points of the pair as the usual breakaway of title contenders at the top fails to emerge.
Chelsea, whose early season form suggested they could be in the thick of the battle, lost 2-1 at 10th-placed Wolverhampton Wanderers on Tuesday -- meaning back-to-back defeats for Frank Lampard's team who lost at Everton on Saturday.
Manchester City, who drew in their local derby at United on Saturday, were left frustrated after being held to a 1-1 draw at home to 19th placed West Bromwhich Albion.
Chelsea and City were in Champions League action last week before domestic fixtures on Saturday and then a quick turnaround for Tuesday's games and Lampard said he felt tiredness had been a factor in his team's lacklustre second-half display.
"There could be some fatigue," said Lampard, whose side have already played five games in 15 days this month.
"I hate to say it after a defeat because it feels like an excuse but even in the first 60 minutes when we were the better team, we didn't have a zip about us.
"We've travelled up to Everton, came back and then travelled to Wolves but everyone is in the same boat. We have a couple of injuries in areas which means I can't make too many changes so I understand that side of it," he added.
City midfielder Ilkay Gundogan, who put his side ahead with a first-half goal before West Brom fought back and held on for a point, said consistency was proving elusive for many teams.
"It's nearly impossible to have just one team that's on top and dominating anything, especially with the schedule coming up," he said.
"All teams in Europe are struggling to win games in a row. But we have responsibilities and we should win these kinds of games, especially at home," added the Germany international.
With the festive season bringing its usual glut of fixtures, Gundogan knows there is no respite in sight.
"We are human beings, not machines, we struggle as well. It is not going to get easier in the next few weeks".
(Reporting by Simon Evans; Editing by Ken Ferris)
