Olivier Giroud came off the bench to score twice as Antonio Conte’s side snatched victory from the jaws of defeat, on what was Mark Hughes’ first home match in charge of the Saints in the Premier League.
Simon Johnson was our reporter at St Mary’s, and here is his analysis of the match.
FA Cup semi-final could be a fun watch after all
Let’s be honest, the FA Cup semi-final clash between these two sides next week is very much after the Lord Mayor’s show as far as most of the country is concerned.
All attention will be on Manchester United’s clash with Tottenham next Saturday, with many expecting the winners of the competition to come from that tie.
Chelsea and Southampton play a day later and it is unlikely neutrals were too desperate to tune in.
But maybe they will change their mind after this remarkable affair which saw Chelsea come from two goals down.
Despite their relegation fears, Southampton looked by far the better side for an hour and certainly looked hungrier for the cause.
Manager Mark Hughes has only recently been appointed at St Mary’s Stadium and it is pretty obvious that his players are doing their utmost for him.
They could be scarred though from letting three points slip, while Chelsea will be boosted by staging such a dramatic comeback.
If these two teams can share five goals again at the national stadium, no-one will be complaining.
Marcos Alonso will be a worried man
The Spain international may not even get to play in the showpiece occasion if the FA decides to take a look at his challenge on Shane Long.
Referee Mike Dean didn’t even award a free kick as the Spain international appeared to stamp on the back of the striker’s calf before the break.
Surely the match official would have considered showing a red if he had seen the challenge?
As is custom, the FA will wait for Dean’s match report to see if he did spot the incident before deciding to take any further action.
If they do deem Alonso’s tackle as violent conduct, then he could be hit with a retrospective three game ban, which will see him sit out the Cup clash.
Any punishment will come too late for Southampton though, given Chelsea were able to carry on playing with 11 men. To make matters worse, Alonso set up Chelsea’s first goal.
Chelsea continue to rue selling Ryan Bertrand
The west London club have been guilty of a lot of strange decisions in the transfer market over the years but allowing Bertrand to leave has to be one of the most odd.
Bertrand came through the club’s academy, infamously made his Champions League debut when they won the final in 2012 and is now an England international.
Remarkably, Chelsea sold him to Southampton for just £10m in 2015, a ridiculously low price for such a talent.
And once again he showed what his old club were missing. Captain of Southampton for the day, he was inspired at left back and set up their opening goal.
The timing of his display in front of the TV cameras didn’t do his chances of making England’s World Cup squad any harm at all either.
Conte has to start Giroud up front in the FA Cup semi-final
Chelsea had toiled with the disappointing Alvaro Morata up front for an hour until Giroud came off the bench.
One suspects Morata has only started the majority of games lately, despite Giroud’s arrival in January, because of his £58m price tag.
Morata has scored a couple of goals lately, yet his all-round game continues to underwhelm.
Southampton defender Maya Yoshida bullied him in the air and on the ground until Conte decided to put the Spain international out of his misery.
In contrast, Giroud showed the hunger and despite you’d expect from a front man and was the man of the match despite playing just half an hour.
The Frenchman’s header was emphatic and his half volley to win the game was sublime. The passionate celebrations showed how much he cared and inspired those around him.
Southampton wouldn’t want to face Giroud again in a hurry which is exactly why he should be on the team-sheet a week tomorrow.
PFA Player of the Year Awards list is telling
This time last year, Chelsea had N’Golo Kante and Eden Hazard vying for the main award, but it is significant that no-one in blue made the list in 2018.
Kante has come close to matching the standards he set in the title-winning campaign, but no-one else came close.
It won’t be surprising if Chelsea players are completely ignored by their fellow professionals when it comes to voting for the PFA team of the Year either.
Now, if the PFA did a season’s worst list, the Blues could be well represented given their dramatic fall from grace.
Southampton: McCarthy, Hoedt, Cedric, Bednarek, Yoshida, Romeu, Hojbjerg, Bertrand, Ward-Prowse, Tadic, Long.
Substitutes: Austin, Lemina, Boufal, Gabbiadini, McQueen, Sims, Forster.
Chelsea: Courtois, Azpilicueta, Cahill, Christensen, Zappacosta, Fabregas, Kante, Alonso, Willian, Hazard, Morata.
Substitutes: Caballero, Barkley, Pedro, Bakayoko, Moses, Giroud, Emerson.
Referee: Mike Dean (Wirral)