Champions Chelsea crashed to their fourth defeat in eight English Premier League football 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, had earlier gone two points clear at the top after Sergio Aguero scored five goals in a crushing 6-1 defeat of Newcastle United.
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.
The win lifted Southampton up to ninth after their first win at Chelsea since 2002.
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.
In the other earlier matches, Crystal Palace beat West Bromwich Albion 2-0 to rise to third on 15 points, level with Leicester City, who won 2-1 at Norwich City.
Sunderland let slip a 2-0 lead to draw 2-2 with West Ham United, Stoke City won 1-0 at two from bottom Aston Villa and Bournemouth drew 1-1 with Watford.
On Monday Everton play Liverpool, Arsenal face Manchester United, who will go back on top with a win at the Emirates and Swansea City play Tottenham Hotspur.