Premier League champions Chelsea's fall from grace continued with a humiliating defeat by lowly Bournemouth on Saturday after Leicester City's improbable title challenge was bolstered by a 3-0 victory at Swansea City.
A late headed goal by substitute Glenn Murray earned Bournemouth a shock victory at Stamford Bridge and left Blues boss Jose Mourinho trying to explain away an eighth defeat in 15 league games since the start of the season.
Leicester moved two points clear at the top after Riyad Mahrez's hat-trick sank Garry Monk's sorry Swansea side.
Their England striker Jamie Vardy's record 11-match scoring streak ended in south Wales but Algerian Mahrez delivered the points as Leicester capitalised on previous leaders Manchester City's dismal 2-0 defeat at Stoke City in the early kickoff.
Mahrez netted twice in the first half and struck again after the break as Leicester moved to 32 points, two clear of Arsenal, who are now second a point above Manchester City after a 3-1 win over Sunderland - Olivier Giroud scoring for both sides.
Two goals inside the opening 15 minutes from Stoke's Austrian striker Marko Arnautovic, both created by Swiss playmaker Xherdan Shaqiri, condemned injury-hit Manchester City to defeat at a blustery Britannia Stadium.
Manchester United drew 0-0 with West Ham United to stay fourth, level on 29 points with neighbours City. There were some boos after a fourth goalless draw in their last six home games.
As role reversals go, the Premier League has never witnessed anything quite like the change in fortunes between Chelsea and Leicester, who last term spent 140 days at the foot of the table while the Londoners were running away with the title.
After 15 games last term, Chelsea were 26 points ahead of Leicester but they now find themselves 17 adrift of them.
Leicester's title odds have gone from a pre-season 2000-1 to 33-1, while Chelsea are three points above the relegation zone in 14th place, 14 points off the top four.
Mourinho again left Spain striker Diego Costa on the bench but even when the Spain striker was introduced to pep up a lethargic Chelsea after the break the hosts could find no way through a resolute Bournemouth rearguard.
Costa's cross clearly, if accidentally, struck the arm of the sprawling Simon Francis but the referee did not give a penalty, much to the animated Mourinho's annoyance.
Then Murray, who had been on for just 90 seconds, headed Bournemouth's winner with eight minutes left.
Chelsea's first Premier League home defeat in their last 44 games against a newly-promoted side deepened the crisis at Stamford Bridge but lifted Bournemouth out of the drop zone.
Mourinho complained about Chelsea not being awarded a penalty and Bournemouth's time-wasting tactics, but the reality is the Portuguese faces a spiralling crisis that could become terminal if they lose to his former club Porto in the Champions League this week and fail to reach the last 16.
Fifth-placed Tottenham Hotspur drew 1-1 at West Bromwich Albion after England midfielder Dele Alli's early strike was cancelled out by a James McClean header at the Hawthorns.
Aston Villa's under-pressure manager Remi Garde is still awaiting his first win with the bottom club, who at least earned some comfort in a 1-1 draw at Southampton.
Villa have six points and are seven adrift of the safety zone, with Bournemouth and fellow strugglers Norwich City, who lost 2-0 at Watford, level on 13 points above Sunderland on 12 and Newcastle United, who host Liverpool on Sunday, with 10.