World number one Rafael Nadal takes on Dominic Thiem on Wednesday as he looks for a record-equalling 20th Grand Slam title at the Australian Open.

Nadal has never lost to Thiem at Major tournaments, beating him in the last two French Open finals, but he's taking nothing for granted against the Austrian fifth seed.

"We know each other well. He's a player that I like a lot, the way that he works, the way that he plays, and the way that he tries his best always," Nadal said.

"It is a match that's going to be a tough one, but will be interesting. I know I have to be at my best to have chances."

Nadal and Thiem are playing for a semi-final against Alexander Zverev or Stan Wawrinka, who won the Australian title in 2014 -- the only player outside the Big Three to do so since 2006.

Now ranked 15, the three-time Grand Slam winner said he's playing his best tennis since knee surgery in 2017, with the passion to win another big event still burning bright at 34.

"I've been feeling great on the court, on the practice court, putting a lot of work in physically and tennis-wise," said Wawrinka, who will have to overturn a 0-2 record against seventh seed Zverev.

"Zverev, he's been in the top 10, top five since few years now. It's going to be a difficult match, for sure."

Elsewhere, Wimbledon champion Simona Halep takes aim at the semi-finals when she plays Estonia's Anett Kontaveit, while resurgent Spaniard Garbine Muguruza faces 30th-seeded Russian Anastasia Pavlyuchenkova.

"I'm not thinking negative. I just want to go there and to give my best," said Halep. "This is all I have in my mind."