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.
But managerless Southampton will hope it will kick-start their attempts to escape the relegation zone.
The visitors had 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.
The half-time introduction of Raheem Sterling breathed new life into the Blues' toothless forward line, but Ainsley Maitland-Niles' crucial block denied the England international an equaliser midway through the second period.
Romain Perraud followed suit minutes later with a tremendous goal-line clearance to prevent a Sterling header from ending up in the net, as Potter's team failed to score for the sixth time in 2023.
There was a lengthy stoppage towards the end of the second half because of an injury to Chelsea captain Cesar Azpilicueta, who was caught in the face by Southampton substitute Sekou Mara.
The 33-year-old needed oxygen treatment on the pitch before being carried off on a stretcher, although he was conscious as he left the field.
There was a minute's applause before kick-off in memory of former Chelsea winger Christian Atsu, who was found dead almost two weeks on from the earthquake in Turkey.
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