Jadon Sancho scored his first goal for Chelsea as they ruthlessly capitalised on a series of Southampton errors to run out comfortable winners.
Sancho wrapped up the scoring for the Blues after goals from Axel Disasi, Christopher Nkunku, Noni Madueke and Cole Palmer had put them in control.
Saints briefly levelled at 1-1 through Joe Aribo but their capacity to self-destruct was underlined when, after falling 3-1 down, captain Jack Stephens was sent off by referee Tony Harrington for pulling Marc Cucurella’s hair after a lengthy VAR check.
With Chelsea flying high and their hosts bottom of the table, there was no doubting who the favourites were going into the match.
And when goalkeeper Joe Lumley failed to clear Enzo Fernandez’s near-post corner, Disasi headed home from two yards out to put the visitors in front.
The lead lasted just four minutes, though, as Joe Aribo made up for squandering an earlier chance by turning home an excellent cross from full-back Kyle Walker-Peters for a deserved equaliser.
But another Lumley mistake – this time while playing out from the back – allowed Nkunku to score in an empty net and restore Chelsea’s lead.
They doubled that advantage just after the half-hour as Madueke’s tricky dribbling took him past Ryan Manning before stroking the ball into the far corner.
And what already looked an uphill battle became even harder for Saints when Stephens was dismissed.
Palmer and Tosin Adarabioyo hit the woodwork as Chelsea continued to dominate – and the second half brought further trouble for Russell Martin’s struggling side.
Joao Felix headed a simple chance wide immediately after the break, Tosin again hit the woodwork in the 53rd minute and Palmer, Madueke, and Nkunku all had chances to extend the lead.
It was Palmer who finally did so, scoring his ninth of the season after Nkunku’s initial effort had been blocked.
Substitute Sancho then opened his Chelsea account by finishing in a one-on-one chance from Malo Gusto’s through ball.