Halep looks all over the place as she plonks three shots wide and gifts Kerber two set points.

The world number one steadies herself to snatch it back to 40-30 and manages to overpower the former champion to bring us to deuce.

Kerber finds another set point and pumps her fist as she watches Halep shoot wide.

What a comeback this is from Angelique Kerber! We're going to a decider.

Image source, Getty Images

Kerber was 3-1 down in this set.

She's about to serve to force a decider.

Image source, EPA

Kerber gets the break!

A long rally, both players hunkered in the corners, and Halep's attempt down the line just had too much on it.

Image source, Getty Images

A loopy forehand from Halep drifts over the baseline.

Break point Kerber...

For the first time in this match, Simona Halep looks a little miffed as successive backhands find the net.

Two furious forehands get her to parity at 30-30 before a lofted forehand from Halep is called out. I reckon that's in. Halep challenges - and it is! The point goes the Romanian's way.

A drop shot finally works for someone, though, as Kerber forces Halep forward and, after a bit of to and fro, Kerber passes Halep.

Piers Newbery
BBC Sport at Melbourne Park

There are days when it feels health-endangering to be sitting on the sunny side of the court at Melbourne Park - notably last Friday when some hardy Brits took in Kyle Edmund's third round match in 41C heat - but on days like today it's the only place to be. Fans are being wafted, legs stretched over empty seats, and sunscreen liberally applied.

Running around is best left to the professionals, and Halep and Keber are certainly covering plenty of ground. Halep has had her chances in this set, with a point on serve for a 4-1 lead, and a break point she put straight into the slot for Kerber's favourite forehand down the line.

An attempt at a down the line strike drifts just long by Simona Halep.

Now here comes Halep with the drop shot and Kerber runs, stretching desperately - and she passes Halep with a winner!

So, so good from both players.

Halep mimes to herself as she works a backhand into the foot of the net.

Advantage Kerber - but she can't make it count as Halep nails a return onto the baseline.

Deuce #3.

Simona Halep has just mis-hit a forehand, and it's still ended up being a winner. Break point number two.

Kerber lashes an uppish forehand, it's called in, Halep challenges - and it is indeed in! Back to deuce we go.

Serve out wide from Kerber, Halep responds, and Kerber tonks a winner down the line.

Deuce!

Angelique Kerber races to a 30-0 lead but her second serve still isn't great, and Halep's power forces her to rush a forehand into the net.

An obscenely good, angled forehand from Halep, taking a big swing and landing the ball on the service line, puts her in control.

Another break point to the Romanian...

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A witty DJ in Melbourne opted to play Stayin' Alive by the Bee Gees during the changeover.

Emphatic from Halep, jogging up to the net and leaping to nail a forehand volley, before sustained pressure draws the error out of Kerber.

Excellent from the world number one. She holds to love.

Image source, Reuters

A deep return from Halep is picked up well by Kerber, and she forces Halep to net a jabby backhand.

Kerber lets out an almighty yell as she gets a loopy ball from Halep and she can thwack it away emphatically.

And Halep goes long on the backhand! She challenges, but to no avail. We're level in the second set.

These two have won four matches apiece against each other.

Kerber has won the last two but Halep's last win against her in 2016 came on a hard court in Montreal.

Time for Kerber to serve.

Image source, EPA

Ooh. Simona Halep gets an angled attempt wrong, and she mutters to herself as Angelique Kerber gets a break back point.

Kerber gets it! She forces Halep to the forehand side and eventually the Romanian nets.

The down the line winner from Kerber is lovely, but her attempt at a drop shot is sent whizzing back past her by Halep for 30-30.

Long goes Kerber on the return of serve, but now it's Halep that shanks a forehand!

This match. Honestly. Deuce!

This has been a stonking performance from Simona Halep.

Can she consolidate the break?

Image source, AFP

Eurgh! A lazy slap of a backhand from Kerber is into the net, and Halep has another chance to break.

And she takes it! Huge hitting from both players and Kerber, sliding and falling around, somehow hangs in until she balloons a forehand out.

Saved with a serve out wide!

Hello, deuce.