Eden Hazard opens the scoring after just five minutes.
Oriol Romeu equalises against former club mid-way through the first 45 minutes.
Gary Cahill heads Chelsea back in front in first half stoppage time.
Diego Costa nets twice including 50th Premier League goal to extend lead.
Ryan Bertrand nets late consolation.
Chelsea go seven points clear of Tottenham Hotspur with victory over Southampton.
Four wins from final five games would guarantee the Premier League title for Antonio Conte's side.
Ninth-place Saints can cement top-half spot with just second win at Stamford Bridge since 2002.
Blues without a clean sheet since January.
Southampton have not beaten a current top-six side all season.
Well that brings an end to our live coverage of this Premier League encounter. We're back tomorrow with three more games including to see if Tottenham can close the gap back to four at Palace. Arsenal host champions Leicester while Sunderland face Middlesbrough. But until then, its goodbye.
Full time: Chelsea 4-2 Southampton
Chelsea go seven points clear of Tottenham and tighten their grip on the Premier League title. Eden Hazard started it all off with the opening, but after Oriol Romeu equalised for Southampton it went all the way of the hosts.
Gary Cahill put the Blues back in front in the twilight of the first half before a Diego Costa double – his first his 50th in the top flight – leading to an easy win. Ryan Bertrand scored very late on but now Tottenham go to in-form Crystal Palace tomorrow night knowing they must play catch-up again.
GOAL! Chelsea 4-2 Southampton (Ryan Bertrand)
Another former Chelsea man gets in on the act in the final of four added minutes as Ryan Bertrand flicks home. But it won't change the result.....
Most popular: Chelsea 4-2 Southampton: Premier League - as it happened
GOAL! Chelsea 4-1 Southampton (Diego Costa)
Goal of the night I would venture. A move involving Hazard and Pedro sees Costa ride two challenges and thump home beyond a woeful Forster. The perfect message to send to Tottenham.
Jay Rodriguez on for Southampton meanwhile. Gabbiadini, who has turned in an odd performance in all, goes off.
Pedro curls wide as Chelsea hunt a fourth. But here comes John Terry for his first appearance since 5 November. He will leave the Blues in the summer of course. It's party time at Stamford Bridge.
Kante almost rounds off the Chelsea win with a goal on the break, but Forster is equal to his dinked effort.
Good pressure from Southampton in the closing minutes here. Looking for a consolation or maybe more. Steve Davis gets one chance from 20 yards but through as crowd of bodies the ball is deflected wide.
And Fabregas gets his deserved ovation from the crowd as he is replaced by Pedro. Into the last 45 minutes.
Cesc Fabregas has been superb tonight. He might not be first choice at Chelsea these days but he can still turn it on when given the chance. The Spaniard has been instrumental in what is going to be a comfortable win.
Dusan Tadic fumes at having a throw-in given against him and he thumps the ball into the ground, earning a yellow card for his troubles. Small spell for Saints but Chelsea looking comfortable it must be said.
Ward-Prowse causes move havoc from a set-piece and Gabbiadini is again at the back post but he slams into the side-netting with his team-mates lining up in the middle. Good chance wasted.
Nathan Redmond on for Sofiane Boufal, who lasts 67 minutes before going off injured. The England international needs to be at his very best to haul this back. Southampton have been totally outplayed since the interval.
GOAL! Chelsea 3-1 Southampton (Diego Costa)
Even with over half an hour remaining, that probably kills it. Chelsea have barely given them a sniff in this second half and they muscle their way into a two-goal lead as Fabregas crosses for Costa who holds of Yoshida and powers beyond Forster.
We're back running again in west London. I have said this many times this season, but a big 45 minutes coming up in the title race.
Gary Cahill is the toast of Stamford Bridge as he puts Chelsea ahead for a second time.
Half time: Chelsea 2-1 Southampton
Chelsea may have had the last word in that first half but they have had it far from all their own way at Stamford Bridge. Southampton did not go away after Eden Hazard's early goal and levelled through Oriol Romeu before going close to taking the lead. Cahill may have put the leaders in the driving seat again but they face a challenge to hold onto that advantage. Better game than you might have expected this one.
GOAL! Chelsea 2-1 Southampton (Gary Cahill)
Chelsea have recaptured their dominance and now they have recaptured their lead. Southampton are forced onto the edge of their six yard box and when Alonso volleys acrobatically across goal Cahill is on hand to thump home a header into the corner.
Southampton celebrate drawing level at Stamford Bridge.
Southampton inches away from taking the lead. Ward-Prowse delivers brilliantly again from a corner and when the ball hits Alonso Gabbiadini throws himself at the ball but can't make contact.
Chances at both ends as this game comes alive. Southampton threaten again as Boufal wins a free-kick and Courtois has to palm away Ward-Prowse's delivery.
At the other end, Kante does justice to his player of the year award with a lung-busting run and cross which is headed away by Saints. But they don't clear and Diego Costa goes down under the challenge of Stephens, but Anthony Taylor waves away appeals for a penalty.
Chelsea have rediscovered their swagger after struggling following the goal.
GOAL! Chelsea 1-1 Southampton (Oriol Romeu)
This was not part of the script. Saints have held their own since going behind and they are level. Chelsea fail to clear a ward-Prowse corner and the ball comes through to Gabbiadini, who actually shoots from the tightest of angles and when Courtois saves with his foot Romeu follows in to net from three yards. What a shocker.
Credit to Southampton they have come back into this game since the goal, having regular forays into the final third, though without being able to test Thibaut Courtois. Claude Puel will be content with what he has seen.
Eden Hazard celebrates sweeping Chelsea into a first half lead.
GOAL! Chelsea 1-0 Southampton (Eden Hazard)
Oh dear, oh dear. Southampton sleepy themselves at the back as Chelsea take an early lead. Diego Costa breaks a non-existent offside line and has all the space and time he needs to tee up Hazard who is unmarked and slots beyond Forster.
Jack Stephens gets on the end of a Southampton free-kick but instead of playing the ball across goal he shoots straight at Courtois.
Chelsea very sleepy at the outset as Cahill and Luiz get in a mess from a long Southampton throw. Gabbiadini causes havoc but Courtois comes out to clear the danger.
Underway in west London.
PFA Player of the Year N'Golo Kante starts again for Chelsea.
Teams on their way out at the Bridge. Chelsea big favourites here. Southampton playing for a few quid and pride mainly.
The Chelsea band are in full voice ahead of kick-off. They have plenty to be happy about, with at the very least an FA Cup final to look forward to.
This week there is an extra dose of Premier League action.
After we are done at Stamford Bridge, tomorrow night Arsenal host Leicester, Tottenham go to Crystal Palace while doomed north east pair Sunderland and Middlesbrough face-off.
On Thursday it is a small matter of the Manchester derby between City and United.
Not bad eh?
Antonio Conte used his post-match press conference after the win over Tottenham at Wembley to attack the Premier League fixture schedule which he feels is unfair on his side. Let me explain.
Southampton did not play over the weekend so have had 10 days between this game and their last, which they lost 3-0 to Manchester City. Spurs meanwhile face a Crystal Palace tomorrow night just three days after they won at Liverpool.
Conte feels this is unfair because not only do Chelsea have one fewer day to recover from the semi-final, but have to face an opponent who has had a significant period of time to prepare for the fixture.
What perhaps the former Juventus coach has not factored in is while Palace are fighting for their lives at the bottom, Southampton arrive at Stamford Bridge will very little - if anything - to play for. And at this stage of the campaign, that often proves vital.
Landmark appearances for Nemanja Matic tonight, after his thunderbolt helped Tottenham at the weekend.
Nathan Redmond an odd omission from the Saints team tonight. Here is potentially why....
Oriol Romeu back on his old stomping ground tonight.
Four changes for the champions-elect Chelsea. Costa and Hazard both return as expected, with Gary Cahill coming back in. The other alteration sees Cesc Fabregas recalled in what looks like quite a narrow Blues offense.
Southampton have had 10 days to prepare for this trip to west London and make just two changes as a result. Steven Davis returns as captain while Sofiane Boufal also starts. Nathan Redmond among those to drop to the bench.
Teams are in from Stamford Bridge.....
Lots of dedications to John Terry in the Chelsea programme tonight, in the first home game since his exit at the end of the season was confirmed. They may regret this when he scores the winner against them next season.
Teams should be confirmed by both sides in the next 20 minutes or so, but until then how do we expect both sides to line up tonight?
Chelsea are expected to recall Eden Hazard and Diego Costa are they were left out of the FA Cup semi-final win over Tottenham. Gary Cahill is also in contention after missing the game at Wembley.
Oriol Romeu, formerly of Chelsea, is back for Southampton after missing two games through suspension. Sam McQueen is out while the fixture comes too soon for Charlie Austin and Matt Targett.
Having ended Tottenham Hotspur's hopes of claiming at least one piece of silverware this season, Chelsea can deal another blow to their London rivals by securing the first of five wins which are needed to deliver the title.
Southampton will be no push-overs however having won at Stamford Bridge last season during Jose Mourinho's tenure. Saints are all-but assured of a top half place under Claude Puel and could be forgiven for already having one eye on the summer with very little to play for. The Blues will be eager to take advantage.
All the team news and build-up ahead of the 7:45pm kick-off on the way in another pivotal night in the Premier League.
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