Substitute Olivier Giroud inspired Chelsea to a 3-2 comeback triumph at St Mary's which dealt a big blow to Southampton's Premier League survival hopes.
Goals from Dusan Tadic and debutant Jan Bednarek looked to have put Saints in control, but Giroud's introduction on 61 minutes proved decisive.
The France striker headed the previously listless visitors back into the game, Eden Hazard levelled shortly afterwards, and then Giroud struck again - all within the space of eight minutes - to spark jubilant celebrations as the Blues drew first blood in this dress rehearsal ahead of next weekend's FA Cup semi-final between the two sides.
Chelsea's lightning turnaround secured just their fourth league win of 2018, boosted their slim hopes of a top-four finish, and denied a stunned Southampton the chance to move out of the relegation zone.
The result denied Mark Hughes a first league win at Saints boss and extended the south-coast club's record home league run without a victory to nine matches.
Chelsea hit half-time having created no genuine scoring chances, failing to capitalise on their dominance at least in the possession stakes.
Saints in contrast buried their sole gilt-edged opening, Tadic capping a flowing field-length counter-attack with a calm finish.
Pierre-Emile Hojbjerg set Ryan Bertrand racing down the left flank, and the England full-back fended off the chasing Cesar Azpilicueta before laying on the perfect cut-back.
The lurking Tadic did the rest, profiting from both his fine positioning and Bertrand's accurate pass to slot home and stun the sluggish visitors.
Antonio Conte's men appeared at best unfazed by Saints taking the lead, but try as they might the Blues failed to kick into gear on the front foot.
Alvaro Morata headed over the bar tamely, while Cesc Fabregas saw a hopeful 20-yard effort take a wicked deflection, forcing Alex McCarthy into a solid save.
Visiting inroads were few and far between beyond that in the first half, however, with Marcos Alonso lucky to avoid a yellow card for a clumsy challenge when seemingly standing on Shane Long's calf.
Chelsea upped the ante after the break, squeezing Saints by playing higher up the field in a bid to break the hosts' double defensive bank.
James Ward-Prowse's raking ball over the top sent Long scurrying clear in a rare breakout, however, only for the Republic of Ireland forward to bungle his eventual shot, allowing Thibaut Courtois to save.
When Gary Cahill failed to cut out Ward-Prowse's whipped free-kick, league debutant Bednarek nipped in at the far post to slide home and double Saints' lead.
But just when the hosts looked home and hosed, Giroud clambered off the bench to tip the game on its head.
The former Gunner buried a testing header to make things interesting, before Hazard rounded off some neat play by blasting home.
Not satisfied with that, however, Giroud then lashed in a ruthless half-volley to floor the shattered hosts.