Croatian Marin Cilic is through to the semi-finals of the Australian Open after No.1 seed Rafael Nadal retired from their quarter-final on Rod Laver Arena.

Nadal received treatment for a leg issue in the fourth set before he succumbed down a break in the fifth set with Cilic leading 3-6 6-3 6-7 (7-5) 6-2 2-0.

"Sometimes in these occasions where a player gets hurt a bit you're not exactly sure what are his possibilities ... how is he going to come up," Cilic said.

"I just tried to stay in my own box and continue with playing well, continue with that intensity. I kept with my own game, with the plan, obviously it paid up.

"In the end I was very unfortunate because Rafa is always fighting really hard, always giving the best on the court.

"He played a very good tournament ... obviously it's very sad for him to finish the way he did."
Cilic has made the semi-finals at Melbourne Park once before, in 2010, when he was beaten in four sets by Andy Murray.

Nadal called for the trainer for the first time down 1-4 in the fourth set and received treatment high on his right leg by the side of the court.

He received further treatment after he lost the fourth set and shook Cilic's hand after he lost his first service game of the final set.

"It's not my hip, but I can't tell you exactly the muscle," a despondent Nadal told reporters.

"It's high on the leg.

"We'll communicate what's going on after I have an MRI (on Wednesday)."

Cilic will meet British surprise packet Kyle Edmund in the semi-finals after he upset No.3 seed Grigor Dimitrov 6-4 3-6 6-3 6-4.

Incredibly, Edmund is just one win away from supplanting three-time major champion Andy Murray as the British No.1.

The 23-year-old now stands just two wins away from what would be a hugely improbable triumph at a tournament where Sir Andy has reached the final five times without ever lifting the title.

Edmund has already created history as the first British man other than Murray to reach the Australian Open semi-finals in 41 years.

Not since John Lloyd in 1977 has anyone but Murray achieved the feat.

"It's an amazing feeling - I'm very happy," said Edmund.

"With these things you're so emotionally engaged that you don't really take it in, you don't really enjoy yourself, so just at the end ... I just really tried to enjoy the moment."

The victory came in Edmund's first-ever match on Rod Laver Arena against a better-credentialled opponent in Dimitrov, who had ended the campaign of local favourite Nick Kyrgios in the previous round.

"It hurts - and so it should," Dimitrov said.

"I need to give myself a couple of days just to relax a little bit and do things I don't get a chance to do during the tournament, and reassess the whole Australian trip.

"Overall it wasn't a bad one, but it certainly wasn't where I wanted to be."

In the women's draw, 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 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.

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'."