Match ends, Chelsea 0, Southampton 2.
Chelsea manager Frank Lampard called on his players to show "personality" after they were condemned to consecutive home defeats for the first time since 2011 by an impressive Southampton.
Lampard's side fell behind to Michael Obafemi's superbly taken strike at Stamford Bridge and had fewer shots than in any game since he took charge during the summer.
The introduction of Mason Mount for defender Kurt Zouma at the break injected some energy but only a vital Fikayo Tomori intervention denied the away side a second.
And with 20 minutes remaining, a fine team move saw the ball break for Nathan Redmond and his simple finish from 10 yards wrapped up six points for Ralph Hasenhuttl's side from festive away games at Aston Villa and the Blues.
It means Chelsea have now lost five of their last seven league games and three times in a calendar month for the first time since 2015.
"We control a lot of the ball but we do not do enough with the ball around the box," Lampard told BBC Radio 5 Live.
"I think it's been the story for us for a while, not just in recent times. We need to get better because we need a better home record.
"What we need to do, the players on the pitch, whoever they are need to make goals and score goals. It's the hardest thing to do in football. It's a challenge for all of us."
Abject Chelsea fail to get going
Hasenhuttl was suitably impressed and raced on to the pitch with his fists clenched on the final whistle after a result which moved his side up to 14th in the Premier League - three points above the drop zone.
Southampton - on 21 points - are six better off than they were at the same halfway stage last season. Lampard's Chelsea are fourth in the Premier League but eight points worse off than they were under Maurizio Sarri after 19 games last term.
Despite coming off Sunday's win at Tottenham, Chelsea found no sort of rhythm and when Pierre-Emile Hojbjerg found Obafemi, he was allowed time and space to progress, shift the ball on to his left foot and fire high into the net from the edge of the box.
Chelsea's Cesar Azpilicueta flicked a corner into the six-yard box - forcing the impressive Alex McCarthy to help the ball away from danger - but it took the home side until the stroke of half-time to find the target with a tame effort from 30 yards.
Mount came off the bench and quickly gave Tammy Abraham the chance to fizz a shot into the side-netting but Chelsea were collectively poor and both Tomori and goalkeeper Kepa Arrizabalaga make key second-half interventions to keep the score at 1-0.
Lampard's side ran over 5km less than the visitors overall and generally lacked creativity and energy. The January transfer window - where they can again sign players after serving a ban - looks key to giving them the chance to inject something fresh and change their recent fortunes.
"We have found now that when teams make it difficult for us and do not give us space, we are finding it tough and so it clearly needs to improve," added Lampard.
"We need to tough it out and get over it quickly. Personality is a huge thing in football - you cannot let that beat you."
Brave Ralph and a brave display
Arsenal and Liverpool were the last sides to record back-to-back wins at Chelsea in 2011 and Southampton deserved to follow Bournemouth in leaving Stamford Bridge with all three points in December.
Hassenhuttl made the brave call to leave Danny Ings on the bench, despite his 11 goals leaving him second to only Jamie Vardy in the top-scorer charts this season.
Obafemi - who had not netted in over a year - and Che Adams - who is yet to score for Saints - worked relentlessly up front for the away side, while James Ward-Prowse was the only player on the pitch to run over 12km in a tireless display.
The midfielder contributed nine tackles - three more than anyone on the pitch - while the Southampton back four repeatedly seemed well positioned, with Jack Stephens and right-back Cedric Soares constantly turning crosses away.
When composure was needed, the away side found plenty and their second goal saw them run the clock down with a lengthy spell of possession that ended with Redmond netting his second league goal of the season.
The fact the ball was turned into his path by a Chelsea player may hide the large number of passes in the build-up, but Saints could then ease home as they ended a run of 13 league games without a clean sheet.
Hassenhuttl will also get the chance to strengthen in January but he looks to have harnessed a superb spirit, a feat that looked unlikely after a 9-0 home defeat by Leicester just 10 games ago. His side have now lost just twice in seven league outings.
"In the first two or three months we played like a relegation team and now we are playing like a Premier League team," said Hassenhuttl afterwards.
Man of the match - James Ward-Prowse
Shots falling for Blues - key stats
- Chelsea have lost seven Premier League games this season - only one fewer than they lost in the entirety of the 2018/19 season under Maurizio Sarri (8).
- Chelsea have lost three Premier League games during December 2019, their most in a calendar month since October 2015 (also three defeats).
- Southampton have won four of their last six Premier League matches (L2), one more than in their previous 20 games (W3 D6 L11).
- Southampton won an away Premier League match against a team starting the day in the top four of the division for the first time since May 2016, a 2-1 win at Spurs.
- Chelsea had just 10 shots against Southampton, their fewest in a Premier League game under Frank Lampard and fewest since April against Liverpool (six).
'Maybe our best game' - reaction
Chelsea manager Frank Lampard: "Teams are making it difficult here. We are not making enough chances, we were at the beginning of the season. I think we have enough to be better than we were today. It needs to be a quick learning process now."
Southampton manager Ralph Hassenhuttl: "I think we saw the passion from every player on the pitch today. It was maybe the best game of the season. With fixtures coming, we needed to rest players but everyone played a fantastic game."
Southampton now have back-to-back home games against Crystal Palace on Saturday and Tottenham on New Year's Day. Chelsea face a trip to Arsenal on 29 December and then go to Brighton on 1 January.
- 15ZoumaSubstituted forMountat 45'minutes
- 29TomoriBooked at 62mins
- 7KantéBooked at 62mins
- 5JorginhoBooked at 58mins
- 20Hudson-OdoiSubstituted forPulisicat 67'minutes
- 10WillianSubstituted forPedroat 84'minutes
- 2Cédric Soares
- 21BertrandBooked at 30mins
- 17ArmstrongSubstituted forRomeuat 86'minutes
- 20ObafemiSubstituted forIngsat 69'minutes
- 10AdamsSubstituted forBoufalat 80'minutesBooked at 90mins
- Jonathan Moss
- Shots on Target
Second Half ends, Chelsea 0, Southampton 2.
Sofiane Boufal (Southampton) is shown the yellow card.
Corner, Chelsea. Conceded by Alex McCarthy.
Attempt saved. Mason Mount (Chelsea) right footed shot from a difficult angle and long range on the left is saved in the top centre of the goal.
Christian Pulisic (Chelsea) wins a free kick on the left wing.
Foul by Oriol Romeu (Southampton).
Mason Mount (Chelsea) wins a free kick in the defensive half.
Foul by James Ward-Prowse (Southampton).
Attempt missed. Mason Mount (Chelsea) left footed shot from outside the box is close, but misses to the right. Assisted by Emerson.
Substitution, Southampton. Oriol Romeu replaces Stuart Armstrong.
Foul by Antonio Rüdiger (Chelsea).
Danny Ings (Southampton) wins a free kick on the left wing.
Substitution, Chelsea. Pedro replaces Willian.
Attempt missed. César Azpilicueta (Chelsea) header from the left side of the six yard box is just a bit too high. Assisted by Emerson with a cross following a corner.
Corner, Chelsea. Conceded by Cédric Soares.
Jorginho (Chelsea) wins a free kick in the defensive half.
Foul by Sofiane Boufal (Southampton).
Substitution, Southampton. Sofiane Boufal replaces Che Adams.
Offside, Chelsea. Mason Mount tries a through ball, but Tammy Abraham is caught offside.