Chelsea capitulated and were booed off as Southampton won at Stamford Bridge for the first time in almost 14 years to leave Jose Mourinho contemplating a fourth loss from eight Premier League games this season.
Captain John Terry was restored but Saints exposed the now familiar failings in the defense which just five months ago formed the foundation for winning the title.
Chelsea have conceded at least two goals in seven of their eight Premier League games this season – the exception the 2-0 defeat of Arsenal – and time and again Sadio Mane, Graziano Pelle and Dusan Tadic found holes in a 3-1 win.
Willian scored a fourth free-kick goal in four games to open the scoring, but Chelsea afterwards scarcely tested Maarten Stekelenburg as they fell to a first home loss to Saints since January 2002.
A rare Steven Davis goal leveled matters and Mane put the visitors ahead after mistakes by Gary Cahill and John Terry, before Pelle got Southampton's third.
Southampton might have had a penalty before the equaliser when Mane was tripped by Ramires.
The forward was soon after booked for diving, while Radamel Falcao received a yellow card for the same offense as Chelsea desperately sought something in attack.
Loic Remy was sent on after Pelle's goal, Nemanja Matic making way just 28 minutes after coming on as a half-time substitute.
But this was not a performance for one scapegoat as Chelsea disintegrated against an excellent Southampton.
Mourinho insisted there were no prima donnas or ostriches at Chelsea and his players accepted criticism, but Terry – making his second start in six games, the other at Walsall – might as well have had his head in the sand when a Tadic corner was floated over towards Jose Fonte.
Had the ball been a foot lower, it would have been 1-0 to Southampton as Fonte had lost Terry six yards out.
Oriol Romeu and Ryan Bertrand seemed keen to impress after being discarded by Mourinho, but both Southampton players were booked, the former for tripping Hazard's heels.
Willian curled the resulting free-kick into the far corner, but Saints – bar two tame Oscar shots – then went on the attack.
Ivanovic was unspotted in tugging back Virgil van Dijk before Davis sliced a good chance wide, and Ramires was fortunate not to concede a penalty for a trip on Mane.
The theatrical fall did not help Mane, who was moments later booked for diving after a tussle with Ivanovic outside of the box.
Fabregas was yellow-carded for diving in last December's fixture at St Mary's and Mourinho was furious there was no penalty award then.
Ronald Koeman had reason to feel aggrieved, but Saints continued to test Chelsea and Asmir Begovic had to be alert to claw away Victor Wanyama's cross and save from Bertrand.
Southampton equalised after Ramires was booked for a foul on Mane, and Fonte played the ball long to Pelle, with his back to goal.
Pelle held off Cahill, chested it and, one bounce later, Davis hit it sweetly to score.
Mourinho almost fell off his seat when Falcao was booked for diving and might have done so again when Cahill lost the ball and Terry failed to intercept Pelle's pass to Mane, who snuck in behind to slot in.
Mane ran at Chelsea and then played in Pelle for the third.
Chelsea tried in vain to find a response, but none was forthcoming in a second Premier League home loss in three games. Mourinho had lost one of his previous 99.
TWEET OF THE MATCH
"@graemelesaux14: I've just turned on the TV against the advice of many of you. Even if I hadn't lost my voice, I'd be lost for words! £cfcvsou" – Former England and Chelsea defender Graeme Le Saux was one of many fans stunned by Chelsea's continued fall from grace.
Asmir Begovic: 6 (out of 10)
Branislav Ivanovic: 4
Cesar Azpilicueta: 5
John Terry: 4
Gary Cahill: 4
Cesc Fabregas: 5
Eden Hazard: 6
Radamel Falcao: 5
Maarten Stekelenburg: 7
Cedric Soares: 6
Ryan Bertrand: 6
Jose Fonte: 7
Virgil van Dijk: 7
Oriol Romeu: 5
Victor Wanyama: 7
Steven Davis: 7
Dusan Tadic: 8
Sadio Mane: 9
Graziano Pelle: 8
Sadio Mane: The Senegal forward continually troubled Chelsea's fragile defence with his pace and movement and scored a deserved goal.
MOMENT OF THE MATCH
There was one foot between Tadic's early corner and the free head of Fonte, who had totally lost marker Terry. Had the ball been a fraction lower, Southampton's captain would have scored, Terry's restoration would have been questioned. It was questioned anyway as Saints stunned Chelsea.
VIEW FROM THE BENCH
Captain Terry was back, along with Eden Hazard. Oscar and Falcao also started despite not travelling to Porto in midweek, the latter more as a result of Diego Costa completing his suspension. Nemanja Matic remained on the substitutes' bench and returned there after a brief appearance in the second half which clearly annoyed Jose Mourinho. Ronald Koeman replaced ex-Chelsea midfielder Oriol Romeu with James Ward-Prowse at half-time and his players repeatedly exposed the Blues' creaking defence.
MOAN OF THE MATCH
Odd coloured boots: If Falcao really needs help identifying his left foot from his right, he really has lost his reputation as a prolific goalscorer.
WHO'S UP NEXT
Chelsea v Aston Villa (Barclays Premier League, Saturday, October 17)
Southampton v Leicester (Barclays Premier League, Saturday, October 17)
After the game, Jose Mourinho suggested his side should have had a penalty when Radamel Falcao went to ground inside the area when the sides were level.
The Chelsea boss suggested the referees were to blame for a slow start to the season which has sparked a confidence crisis within the club.
Speaking to Sky Sports after the game, Mourinho said: "When you are so down it is time to be honest and to say clearly the referees are afraid to give decisions to Chelsea.
"The result (score) 1-1 is a huge penalty and once more we don't get and I repeat, if the FA wants to punish me they punish me, it is not a problem for me."
He went on: "We have a penalty, and the penalty is a giant penalty and he was afraid to give – like everyone is afraid to give – a penalty.
"And after that the team lost even more confidence.
"Psychologically the team is unbelievably down, it looks like good players are bad players.
"The first half was again where we didn't show our quality but we were more than in control and one mistake, lack of concentration and one goal and and you are in a tough time.
"In normal circumstances you come into the second half you do your game, no panic.
"This team need to finish the first half winning two or three nil, with the fear disappearing into the second half to play with a free spirit.
"That's what the team needs but it is not happening."
"The second goal is an individual mistake, the third goal is an individual mistake and the team mentally tried, but they collapsed because they are in such a low moment," he added.
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