Chelsea's poor form under manager Graham Potter continued as a trademark free-kick from James Ward-Prowse earned the Premier League's bottom club Southampton a surprise victory at Stamford Bridge.
Ward-Prowse scored the game's only goal in first-half stoppage time, sending a curling effort over the Chelsea wall and into the corner from the edge of the penalty area.
The result means Chelsea have now won just two games in their last 14 in all competitions.
Managerless Southampton will hope it can kick-start their attempts to escape the relegation zone.
The visitors started magnificently in west London and twice went close to scoring before Ward-Prowse's strike, with Kamaldeen Sulemana shooting straight at Kepa Arrizabalaga before Kalidou Koulibaly blocked Stuart Armstrong's follow-up.