Spain's Rafael Nadal has been in ominous form up till now but faces a high hurdle in the sixth-seeded Marin Cilic. Photo: AFP

Big-hitting Marin Cilic stands in the way of a second successive Australian Open semi-final for Rafael Nadal Tuesday as Caroline Wozniacki looks to make the last four after a seven-year gap.

Top seed Nadal has been in ominous form up till now but faces a high hurdle in the sixth-seeded Croat, a former US Open champion.

They are the night match on Rod Laver Arena with Wozniacki taking on Spain's Carla Suarez Navarro immediately afterwards as the stakes are raised in quarter-final action.

Nadal, who lost to Roger Federer in last year's final, has a 5-1 career win record against Cilic, but is taking nothing for granted.

"I know I'm going to have a tough opponent with Cilic. I need to play aggressive and play well," said the 16-time Grand Slam champion, who has only won once in Australia, in 2009.

"We know each other. We've played a couple of times and I know I need to play well. I hope to be ready to make that happen."

Whoever gets through will face either Bulgaria's world number three Grigor Dimitrov or British hope and 49th-ranked Kyle Edmund in the last four on Thursday.

Dimitrov beat Edmund only two weeks ago in the quarter-finals at the Brisbane International, and is prepared for another struggle.

"I am finding that extra gear in every match and the main thing for me is to really focus on myself," said the third seed, who is gunning for a maiden Grand Slam crown.

Dane Wozniacki last made the semi-finals at Melbourne Park in 2011, and, like Dimitrov, is yet to win a Grand Slam.

She faces a familiar foe in Suarez Navarro, whose two wins in seven career meetings with her were both on clay.

"Obviously hard courts are a little different," said Wozniacki.

"But we've had a lot of tough encounters on hard courts as well. Three-set gruelling matches. I'm expecting a tough fight."

The winner will play either Ukraine's fourth seed Elina Svitolina or unseeded Belgian Elise Mertens.

Svitolina is on a nine-match win streak after winning the Brisbane International to kick off her season, but is not underestimating Mertens, who entered the tournament after winning in Hobart.

"You know, she's been playing well, definitely," she said.

"I didn't see so many of her matches here, but if she's in quarter-finals, definitely she deserves to be there. There is no easy ways to get into a quarter-final."