Marcos Alonso’s brilliant goal gave Chelsea victory at Stamford Bridge.
Alonso scored with a 30-yard free-kick in the final seconds of the first half, superbly bending the ball into the bottom corner of the net.
The breakthrough came shortly after Pedro went close for the Blues when he burst in from the left and fired against the near post.
After coming on as a substitute, former QPR striker Charlie Austin was twice denied an equaliser by Chelsea keeper Thibaut Courtois after being through on goal, first by Pierre-Emile Hojberg and then by Sofiane Boufal.
But Chelsea were largely comfortable and Saints keeper Fraser Foster produced a number of saves, including when he kept out Alonso’s right-footed volley.
Forster also stopped a shot from Alvaro Morata soon after the Spaniard had come on for Eden Hazard.
Chelsea again started without an out-and-out striker, with Hazard leading their attack until Morata’s introduction.
Chelsea: Courtois; Azpilicueta, Christensen, Cahill; Moses (Zappacosta 82), Kante, Bakayoko, Alonso; Willian, Hazard (Morata 73), Pedro (Fabregas 68).
Subs not used: Caballero, Rudiger, Drinkwater, Batshuayi.