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.
With Chelsea flying high - victory took them second above Arsenal - and their hosts bottom of the table, there was no doubting who the favourites were going into the match.
