Good from Wozniacki! She comes charging to her right and angles a forehand back down the line to win the point.

And the Dane wraps things up with an ace. She holds.

Image source, EPA

Elise Mertens gets pushed out wide and she lines up a meaty forehand, but the ball clunks into the net tape and doesn't make it over.

Wozniacki can't make the advantage count, though, as she loosely sends a forehand into the net. Deuce number three arrives.

Hmm. Wozniacki's forehand goes wonky again, as she goes cross court and ends up slapping the ball between the tramlines.

Break point to Mertens - and it's saved with a backhand that goes spinning down the line. Fine work from the Dane.

Better from Wozniack, as a bigger forehand overpowers Mertens, before she crashes an ace down the T for 40-0.

Ooh. Another double fault gets Mertens back into things, and then Mertens launches into a forehand winner! We're at deuce.

Caroline Wozniacki is only 33% on first serve at the minute. She'll need to make those count against Mertens, who looks to be taking the serve on.

Image source, AFP

Piers Newbery
BBC Sport at Melbourne Park

Wozniacki is apparently hitting her backhand 7kph faster and her forehand 4kph faster, on average, than at last year's Australian Open. That is a big difference.

Always one of the great defenders, the Dane has lacked the attacking punch to grab the biggest titles - until recently. Her WTA Finals win last October was the best of her career, and the number one ranking and a first major title are now within her sights. Hard to think of a potentially bigger few days in her career.

And there it is!

Another loose forehand from Elise Mertens finds the net, and the first break of the match goes Wozniacki's way.

There's space there for Elise Mertens as Caroline Wozniacki comes up to the net, but she shanks her forehand into the net.

Again, she gets Wozniacki on the move, but two break points come the Dane's way as Mertens puts too much onto a slice.

Image source, Getty Images

Britain's Andy Lapthorne (above) has gone down to a heavy defeat in the quad wheelchair singles, losing 6-2 6-1 to Australia's Heath Davidson.

He's now lost two of his three round-robin matches with one more to come.

Lapthorne will be in quad wheelchair doubles action later, partnering American David Wagner in the final against Davidson and fellow Aussie Dylan Alcott.

Gordon Reid and Alfie Hewett team up in the men's wheelchair doubles, taking on Argentina's Gustavo Fernandez and Japan's Shingo Kunieda.

Lucy Shuker plays alongside South Africa's Kgothatso Montjane in the women's wheelchair doubles against Dutchwoman Marjolein Buis and Japan's Yui Kamiji.

A stonking service return from Elise Mertens, standing tall and thudding a backhand winner, kicks off the game, before Wozniacki coughs up a double fault.

She's, er, not best pleased, but a few overworked shots from Mertens get her to parity at 30-30. A fine backhand volley, feet off the ground, see Wozniacki through.

A forehand spanked into the net is the only mis-step for Elise Mertens on serve.

She drags Wozniacki back and forth behind the baseline before crashing a forehand winner beyond her. Lovely stuff.

Bang. An ace out wide to start for Caroline Wozniacki.

She gets herself to 30-0 before some nice work from Elise Mertens, first down the line and then a smash at the net, brings things level.

Mertens forces Wozniacki to come forward, and a backhand volley trickles off the net tape to allow the Dane to hold.

A last gulp from the drinks bottles for the players, and then they march out onto court.

It'll be Caroline Wozniacki to get things going.

Elise Mertens and Caroline Wozniacki have met once before - it was the Dane who triumphed then, with a three-set victory on clay in Bastad last year.

Piers Newbery
BBC Sport at Melbourne Park

Image source, BBC Sport

It's an absolute peach of an afternoon at Melbourne Park, 24C and perfect for a stroll.

That is clearly what Chung Hyeon thought when he decided to amble through Garden Square, much to the delight of a group of South Korean fans. It is possible that this whole scenario was arranged by a sponsor, but Chung looked to be having far too much fun for that.

He really has become something of a star over the last few days, with his tennis, his chat and his glasses all grabbing the attention of the locals. If he beats Roger Federer tomorrow night, we will officially be in the grip of Chungmania.

Image source, BBC Sport

Big cheers for Elise Mertens as she makes her way out onto court, followed by Caroline Wozniacki.

Elise Mertens wins the coin toss, and asks Wozniacki to serve first. Wozniacki also gets a brief reprimand from the umpire for being a bit late to the toss.

We could end up with a new world number one at the end of today.

Should Caroline Wozniacki reach the final, and Simona Halep loses her semi-final match, Wozniacki will take the top ranking for the first time since 2012.

If both Halep and Wozniacki win, then whoever triumphs in the final will be crowned number one.

Image source, Getty Images

No matter what happens today, this is Mertens' best run in Melbourne. She fell in the second round of qualifying in her first appearance in 2016.

Wozniacki is making her seventh Grand Slam semi-final appearance, and her second in Melbourne. On the previous occasion she fell 3-6 7-5 6-3 to eventual champion Li Na.

Woz admits it's a loss that still haunts her - she had match point in the second set but couldn't close it down. She's still searching for her first Slam title.

Image source, Reuters

Elise Mertens, unbeaten in all of her 10 singles matches this year, has yet to drop a set in Melbourne.

Making her main Australian Open draw debut, Mertens beat Viktoria Kuzmova and home favourite Daria Gavrilova in the first two rounds.

Alize Cornet and Petra Martic were both dispatched, before Mertens registered her first victory over a top-five player.

She thrashed fourth seed Elina Svitolina 6-4 6-0 in 73 minutes on her way to the last four.

Image source, Getty Images

Caroline Wozniacki ended 2017 in strong fashion as she won the season-ending WTA Finals, and picked up where she left off as she reached the final of the Auckland Open in January.

Seeded second in Melbourne, she was forced to save two match points against Jana Fett in the second round, before closing out a three-set win.

She dispatched Kiki Bertens and Magdalena Rybarikova in straight sets to set up a quarter-final meeting with Carla Suarez Navarro.

Despite breezing through the first set, Wozniacki lost the second-set tie-break, before bouncing back to beat Navarro 6-0 6-7 (3-7) 6-2.

Image source, Getty Images