|Arsenal's Cesc Fabregas, center, celebrates his goal against Chelsea with Theo Walcott, left [Reuters]
Chelsea's worrying slump turned into a full-blown crisis when the Premier League champions were comfortably beaten 3-1 by Arsenal at The Emirates on Monday.
Goals from Alex Song, Cesc Fabregas and Theo Walcott in a nine-minute spell either side of halftime condemned Carlo Ancelotti's team to another defeat and they have now managed just six points from the last 24 on offer.
Branislav Ivanovic replied for the visitors but the damage was already done as Arsenal ended a run of five consecutive defeats against their London rivals to reclaim second place in the table above Manchester City on goal difference.
"The win had a double impact. First of all it keeps us in touch with the leaders and, secondly, it is the
pyschological impact because we were questioned about our ability to win big games," Arsene Wenger, the Arsenal manager, told reporters.
"We know that to beat the big teams at home will have an important impact on the league [title race]."
Arsenal have moved to 35 points from 18 games with unbeaten leaders Manchester United on 37, having played a game less. Chelsea, who started the season in scintillating form but have forgotten how to win, remain in fourth place on 31.
Defeat means Chelsea are 10 points worse off than at this stage last season with their title chances disappearing fast.
Top of the Premier League, still unbeaten and starting to play with their familiar swagger, Manchester United's season could be about to get even better with Wayne Rooney rediscovering his spark.
The England striker did not score in the 2-0 victory over Sunderland on Sunday, but as United prepare for their trip to Birmingham City on Tuesday, there are signs that Rooney is emerging from the worst slump of his career.
To have established themselves so firmly at the top of the league despite Rooney scoring just once in the league this season is a testament to United's strength in depth and their defensive solidity.
Manchester City are emerging as a genuine title threat, having bounced back from their home defeat by Everton to beat Newcastle United 3-1 on Sunday.
Tottenham Hotspur complete the top five after a 2-1 victory at Aston Villa on Sunday and are on a nine-match unbeaten run in all competitions.
With a home game against Newcastle on Tuesday followed by the visit of Fulham on New Year's Day, Harry Redknapp's side could enter the second half of the season in the mix for a "top four" slot or better.
West Ham United's under-fire Avram Grant and Wigan Athletic's Roberto Martinez both enjoyed three points on Sunday but know survival will need a run of decent results.
Grant's Hammers host Everton on Tuesday when victory could take them out of the bottom three while Wigan host Arsenal on Wednesday.
Free-falling Fulham go to Stoke City while Wolverhampton Wanderers, who are now bottom after losing to Wigan, face Liverpool away on Wednesday.