Following a pre-match lights show at St Mary’s, a much-changed Chelsea side put on a display of their own during an entertaining first half.
The visitors raced into a lead with seven minutes on the clock when Axel Disasi’s header helped Enzo Fernandez’s corner over the line.
However, having yet to lose a Premier League game in which they have broken the deadlock, the Blues were pegged back just four minutes later as Kyle Walker-Peters got the better of Fernandez before squaring to Joe Aribo for a simple finish.
Defensive mistakes have plagued Southampton’s season, and yet another cost them parity here when Joe Lumley’s cheap turnover allowed Noni Madueke to pick out Chelsea’s top scorer Christopher Nkunku, who side-footed into an empty net - the Saints’ ninth league error leading to a goal this season.
The Blues were soon knocking down the Southampton door, striking the post through Cole Palmer and clipping the bar via Tosin Adarabioyo before Madueke netted their third, coaxing his way into the box to stroke an effort into the far corner.
Southampton’s uphill task was then made even harder before the break, as captain Jack Stephens was shown a red card for the second time this season, dismissed for pulling Marc Cucurella’s hair.
A fast start to the second period saw Joao Felix somehow head wide from Palmer’s clipped cross, shortly before Tosin smashed a corner delivery onto the post, with Madueke then denied by a last-ditch challenge.
The hosts had a rare chance to claw their way back into the contest when Filip Jorgensen thwarted Mateus Fernandes, but they were soon on the back foot once again as Madueke forced another save from Lumley.
Firmly second-best for the majority of the contest, Southampton’s second-half resolve was eventually broken in the 77th minute, thanks in large part to Nkunku, who surprisingly elected not to shoot when through on goal, instead working the ball to Palmer for a tap-in.
The Blues rounded off an excellent evening with a fifth name on the scoresheet, as Jadon Sancho stayed onside to lash home his first goal for the club.
Three points lift Chelsea into outright second and clear of Arsenal, at least temporarily, adding further fuel to suggestions they may now be in a title race. Meanwhile, no side on five points at this stage of a PL campaign has ever survived - a damning statistic for Russell Martin’s Southampton side who sit rock-bottom of the league with just one win to their name.