Reigning champions Chelsea crashed to their fourth defeat in eight Premier League matches this season when they were beaten 3-1 by Southampton at Stamford Bridge after a woeful performance that left them 16th in the table.
Manchester City, tipped at the start of the season to be Chelsea's main title challengers, went two points clear at the top after Sergio Aguero scored five goals in a crushing 6-1 defeat of Newcastle United.
7 Jamie Vardy (Leicester City)
6 Sergio Aguero (Manchester City)
5 Callum Wilson (Bournemouth)
Riyad Mahrez (Leicester City)
Odion Ighalo (Watford)
Chelsea took the lead through a Willian free-kick after 10 minutes but Southampton struck back with goals from Steven Davis after 43 minutes, Sadio Mane who capitalised on a mistake from recalled skipper John Terry after 60 minutes and Graziano Pelle 18 minutes from time.
While City have 18 points, Chelsea have just eight and are only four points better off than Aston Villa who are in the relegation zone.
Earlier, City, beaten by West Ham United and Tottenham Hotspur in their last two league games, appeared to be in danger of another defeat after Aleksandar Mitrovic put Newcastle ahead in the 18th minute at the Etihad Stadium.
The visitors led until Aguero burst into life with goals after 42, 49, 50, 60 and 62 minutes before he was denied the chance to become the first player to score six in a Premier League match when Manuel Pellegrini substituted him two minutes after his fifth goal.
Kevin de Bruyne also got on the scoresheet for City during Aguero's rampage.
|Aguero had scored just three goals in this season before this game [EPA]
The victory lifted City on to 18 points, two ahead of Manchester United who play at Arsenal on Sunday.
Crystal Palace, who beat West Bromwich Albion 2-0 in the early kickoff, are third on 15 points, level with Leicester City who won 2-1 at Norwich City.
Sunderland, who like their north east neighbours Newcastle United are still without a win, drew 2-2 with West Ham United after being 2-0 up but at least climbed off the bottom of the table with Newcastle replacing them.
Stoke City won 1-0 at third from bottom Aston Villa and Bournemouth drew 1-1 with Watford whose keeper Heurelho Gomes saved a late penalty.
Chelsea 1-3 Southampton
Bournemouth 1-1 Watford
Aston Villa 0-1 Stoke City
Crystal Palace 2-0 West Brom
Manchester City 6-1 Newcastle United
Norwich City 1-2 Leicester City
Sunderland 2-2 West Ham