Chelsea’s defensive shortcomings were evident again and cost them two points.
Timo Werner and Kai Havertz scored their first Premier League goals but an injury-time equaliser by Jannik Vestergaard rescued a point for Southampton.
Two superb first-half goals from Werner put Chelsea two up and seemingly in control.
But Danny Ings pulled a goal back for Southampton shortly before the interval and another appalling error by Kepa Arrizabalaga led to Che Adams equalising in the early in the second half.
Havertz, who gave the ball away in the build up to the Saints’ first goal, restored Chelsea’s lead just a couple of minutes after Adams’ leveller.
However, more shoddy defending from Chelsea was punished in the closing stages.
A needless foul by Kurt Zouma resulted in a free-kick which they failed to deal with and Theo Walcott’s volley was diverted into the net by Vestergaard.
Chelsea went ahead on the quarter-hour mark – shortly after Werner headed in Ben Chilwell’s cross but was denied a goal by an offside flag.
The German left no room for doubt with his beautifully-taken opener.
He collected Chilwell’s ball forward, turned away from Jan Bednarek and evaded two more challenges before firing into the bottom corner.
His second goal was excellently taken too.
Jorginho lifted the ball over the top and Werner drifted away from Bednarek before lifting the ball over McCarthy and heading into an empty net.
It was one-way traffic before Havertz lost possession and Adams played the ball through to Ings, who rounded Kepa and slotted home.
A woeful defensive mix-up gifted the visitors another goal.
The much maligned Kepa, back in the side because of an injury to recent signing Edouard Mendy, made a half-hearted attempt to deal with a poor back-pass from Zouma.
The Spaniard made no contact with the ball and was beaten to it by Ings, resulting in Adams eventually blasting into the net.
Chelsea hit back in emphatic fashion.
Christian Pulisic’s lovely ball released Werner, who squared it for Havertz to score from close range.
Havertz missed a late chance with a header from a corner by Hakim Ziyech, who came on as a second-half substitute for his Blues debut.
And Southampton made the most of that reprieve.
Chelsea: Kepa, Azpilicueta, Christensen, Zouma, Chilwell, Kante, Jorginho, Mount (Ziyech 72), Havertz, Pulisic (James 87), Werner (Abraham 90).
Subs not used: Caballero, Tomori, Giroud, Hudson-Odoi.