Cuco Martina slings in a cross from the Saints right, looking for Steven Davis's forehead. David Luiz arrives at the scene first, however, heading over his own bar.

Chelsea's organisation has been excellent. You could draw a straight line through their back five and midfield four when they're back defending.

When the opposition centre-half is allowed space in your box to bring down a high ball, the signs aren't good. Gary Cahill, up for a free-kick, does that when the ball is pumped into the area for a second time. He finds Victor Moses but he is crowded out.

Image source, AFP

Steve Claridge
Ex-Leicester striker on BBC Radio 5 live

It just seems as though Southampton know what Hazard is going to do, but they still can’t stop it.

If you’re going to defend like you are in numbers, you’ve got to have something that threatens the opposition and Chelsea have that in abundance.

Another minute, another Chelsea chance. Eden Hazard again at the heart of things, shooting low at Saints keeper Fraser Forster.

Forster's shovel-like hands can't scoop up the ball, but his defence clear before a Blue shirt pounces.

Image source, AFP

I feel like there are more goals in this one. Chelsea goals.

Steve Claridge
Ex-Leicester striker on BBC Radio 5 live

It’s not been a poor performance by Southampton. It’s just been the archetypal way to play the game in this system when you go ahead.

As Chelsea have gone ahead they’ve got men behind the ball, but they have that ability on the break to hurt you. It doesn’t need three against three, they get hold of the ball and they can almost go past players at will.

There’s always a threat with Chelsea.

We should have a game on our hands again here. Southampton spurn a great chance. 

A cross from left-back Ryan Bertrand next to the left corner flag lands on the head of Charlie Austin. The in-form Saints striker climbs above Gary Cahill but can't keep his header down. 

Head in hands time. He knows.

Image source, Getty Images

Are Chelsea bona fide title contenders then? I reckon so. No midweek European exertions could be a benefit too.

Alistair Bruce-Ball
or BBC Radio 5 Live Sport

Southampton need something magic, because at the moment everything they’ve done has just run into a blue brick wall.

It needs Southampton to score a goal here to lift it. 

Just seen another replay of that Diego Costa goal. Quality. Started going towards Dave in Row A, before curling deliciously back inside the post.

And that's only the second Premier League goal he has scored from outside the box.

Love of Football: What a strike Costa! 4 points for my fantasy league hard to believe these are the same players from last year.

Graham Green: Right on schedule, Southampton begin their annual mid-first-half-of-the-season swoon.

Shravan Karthik: What a beauty by Costa! Incredible! Can't stop scoring at the moment, Hazard and Costa on fire! Chelsea usually do well!

Mr Godwin The Pundit: Really poor defending from the Southampton right back to allow Costa curl that effort in. Sloppy defending.

Can Southampton recover? Could Hazard and Costa make all the difference in the title race this season? Let us know your thoughts at #bbcfootball on Twitter. 

How is it not 3-0?! Only Diego Costa knows. Victor Moses' stinger is parried by Fraser Forster and Eden Hazard, quicker than a flash of lightning, is on to it. He squares to Costa, who passes instead of shooting, before Pedro is forced to stick on the reverse lights. Chance gone.

Image source, Getty Images

Claude Puel has seen enough. Time for a change. The Saints boss turns to Morocco attacker Sofiane Boufal - the club's record signing - as Jordy Clasie sees his number being held up.

Image source, PA

Alistair Bruce-Ball
for BBC Radio 5 Live

This is the Eden Hazard of a couple of years ago.

Conte has got him in the right position, playing at his best.

And it is almost a third for Diego Costa! His first-time shot flies past the frame of Southampton's goal following neat build-up play between Eden Hazard and Victor Moses.

That would have been - if it is not already - game over.

Steve Claridge
Ex-Leicester striker on BBC Radio 5 live

That’s a fantastic finish.

That’s not great defending there. If I was the manager I wouldn’t be happy with that defending - he should have shut him down, but that is a fantastic finish. Top-draw.

Hazard and Costa have been revitalised this season, and the difference that they make is incredible.

Image source, Reuters

Fantasy Football managers up and down the land rejoice. That's Costa's ninth goal goals in his last 11 Premier League games.

Image source, AFP

Groans from the Southampton crowd as Diego Costa tumbles under Jordy Clasie's heavy tackle. Free-kick inside the Saints half... 

Image source, Getty Images