Olivier Giroud scored twice as Chelsea netted three times in the space of eight minutes to come from two down and win at St Mary’s.
After Giroud, on as a substitute, pulled a goal back for Antonio Conte’s side, Eden Hazard levelled and Giroud then scored the winner.
Former Chelsea man Ryan Bertrand set up a first-half opener for the Saints, speeding past Cesar Azpilicueta and delivering a fine cross to Dusan Tadic, who slotted past keeper Thibaut Courtois.
Worse followed for the Blues when Jan Bednarek, on his Premier League debut, scored from close range after being found unmarked by
James Ward-Prowse’s right-wing free-kick.
Conte responded by sending on Giroud along with Pedro – and the French striker hauled the visitors back into the game by heading home Marcos Alonso’s cross 70th-minute cross – his first league goal for the Blues.
Hazard equalised four minutes later, controlling Willian’s ball into the box and then shooting past former QPR keeper Alex McCarthy.
And Giroud then struck again, firing through a crowd of bodies after Southampton were unable to clear Hazard’s ball in from the left.