Second seed Caroline Wozniacki has advanced to the Australian Open semi-finals with a three-set win over Carla Suarez Navarro.

Wozniacki jumped the unseeded Spaniard early then lost her way in the second set before closing out a 6-0 6-7 (7-3) 6-2 win on Rod Laver Arena on Tuesday night.

The Danish star spurned the opportunity to wrap up the match in the second set when she failed to capitalise on a match point.

"I was unlucky not to get that match point to close it out, which was frustrating," Wozniacki said.

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"But I just had to get myself together and stay a bit closer to the baseline in the third set.

"I felt like I had a little bit more energy than she did in the end."

The match started just after 11.30pm local time and didn't finish until after 1.30am.

Wozniacki was still speaking to reporters an hour after that but she had no issue with the scheduling of the match that followed the Rafael Nadal-Marin Cilic quarter-final.

The 27-year-old said she has treated every match at Melbourne Park as a plus since she staved off two match points to come from 1-5 down in the third set in her second-round encounter with little-known Croatian Jana Fett.

She will attempt to banish the demons of her previous visit to the semi-finals at the Australian Open in 2011.

"Last time I was in the semi-finals here I had match points against Li Na and lost it - that's still haunting me to this day," she said.

"I usually forget matches and I don't remember playing certain people. I don't remember a lot of things, but that one is one that I remember very well."

Denmark's Caroline Wozniacki, left, shakes hands with Spain's Carla Suarez Navarro. Photo / AP Photo
Denmark's Caroline Wozniacki, left, shakes hands with Spain's Carla Suarez Navarro. Photo / AP Photo

Wozniacki will play unseeded Belgian Elise Mertens for a place in the final.

Playing for the first time in the Open, the unseeded Mertens made her maiden grand slam semi-final by shocking world No.4 Elina Svitolina 6-4 6-0 on Rod Laver Arena on Tuesday.

Svitolina was a pre-tournament title favourite, having won nine matches straight to open her summer, and was in line to take over the world No.1 ranking with success at Melbourne Park.

Ranked outside the top 100 12 months ago, Mertens said she wouldn't have believed it if told pre-tournament that a final-four berth was hers.

"I'd laugh a little bit," said Mertens, who had never previously beaten a top- five opponent or made it past a grand slam third round.

"Everything is possible in tennis but of course semis is 'wow'."

On the back of her second Hobart International title, the 22-year-old produced fearless tennis, hitting 26 winners in a dominant display.

Hampered by a hip injury, Ukrainian Svitolina, who is one of the best movers on tour, was unable to respond with her service game particularly affected.

A devastated Svitolina, who despite her ranking was also making her first appearance in a grand slam fourth round, said she had been restricted in her play and training since winning the Brisbane International.