Michael Obafemi and Nathan Redmond were on target as Southampton consigned Chelsea to a shock 2-0 defeat at Stamford Bridge.
Academy product Obafemi, starting in place of the rested Danny Ings, gave Southampton the lead when he curled home a stunning left-footed effort from the edge of the box (31) after Chelsea had lost possession in midfield.
It was the 19-year-old's first Premier League goal in a year and things went from bad to worse for Chelsea when Redmond finished off another slick Southampton counter-attack (73) in the second half.
Frank Lampard's men dominated possession throughout but showed none of the cutting edge that helped them beat Tottenham last time out, allowing Southampton to claim a deserved win which lifts them up to 14th in the Premier League table.
How Saints stunned sluggish Chelsea
Chelsea came into the game in buoyant mood following that excellent win over Spurs, but they looked sluggish from the start and struggled to create meaningful chances, with Fikayo Tomori sending a tame header straight at Alex McCarthy with their only effort on target in the first half.
A string of promising attacks broke down due to poor crosses and a lack of composure in the final third, with the home fans showing their frustration in the stands. But Southampton also defended superbly, with Jan Bednarek and Jack Stephens marshalling Tammy Abraham closely.
The visitors were a threat on the counter-attack too, and it was from one such situation that they took the lead, springing forward after Callum Hudson-Odoi was dispossessed under the nose of the watching Lampard. Within a few seconds, Obafemi was celebrating a memorable goal.
Lampard threw on Mason Mount for Kurt Zouma at the break, changing his system from a back three to a back four, and the substitute made an immediate impact, teeing up Abraham, who fired a diagonal effort into the side-netting just seconds after the restart.
But Southampton defended resolutely and continued to look dangerous on the break, with Redmond's pace and directness causing Chelsea serious problems.
In fact, the winger should have doubled Southampton's lead before he did, running through on goal in the 66th minute only to be denied by Kepa Arrizabalaga's low save.
He made amends not long afterwards when he clipped a cool finish over the Chelsea goalkeeper after N'Golo Kante had inadvertently guided the ball into his path, and the hosts were unable to recover. By the end of the game, they had only managed three shots on target all afternoon.
The result makes it two consecutive home defeats against lower-half opposition for Lampard's side following the 1-0 loss to Bournemouth. They now sit just three points ahead of fifth-placed Tottenham in the Premier League table.
Chelsea face Arsenal live on Sky Sports Premier League from 1pm on Sunday at the Emirates Stadium (kick-off 2pm), while Southampton are at home to Crystal Palace on Saturday, kicking off at 3pm at St Mary's Stadium.