German stars Timo Werner and Kai Havertz scored their first Premier League goals for Chelsea, but there were familiar failings in the home team’s ranks and Frank Lampard’s men were punished.
Chelsea were thrilling going forward at times, with Werner, who also had a goal disallowed, netting twice before setting up Havertz for what looked set to be the winner.
But between Werner’s double and Havertz’s strike it had been the same story for the Blues’ defence, with Danny Ings and Che Adams taking advantage, and Walcott’s shot from the edge of the penalty area in stoppage time was nodded in by Jannik Vestergaard to pinch a point.
Chelsea were two clear after 28 minutes as Werner raced onto Jorginho’s pass, held off Bednarek and flicked the ball coolly over McCarthy before heading in.
Ings ran through a dozing defence to haul Southampton back into contention in the 43rd minute, and a Chelsea calamity came early in the second half.
The error-prone Kepa pulled out of a challenge to meet an under-hit backpass from Kurt Zouma, and that allowed Adams to win the ball and thrash in from close range.
Havertz turned in a short pass from Werner to nudge Chelsea 3-2 in front two minutes later, but there was a sting to come.
A Southampton free-kick was only half-cleared and Walcott – a teenage star at St Mary’s who has returned on loan from Everton – lashed a strike goalwards, with Vestergaard applying the decisive touch.
Werner had given Chelsea a 15th-minute lead when he burst from the left channel, selling a dummy to Jan Bednarek before taking a circuitous route around a gaggle of defenders and firing low beyond Alex McCarthy.