Melbourne (AFP) - Tough American Sofia Kenin stands between Garbine Muguruza and a third Grand Slam title in Saturday's Australian Open women's final after a tournament of twists and turns that has defied all expectations.

Few would have predicted that either Kenin or Muguruza, who has spent the past six months in a tailspin, would reach the final but they have fought their way through to stand one win from glory.

The feisty Kenin shocked Australia's world number one Ashleigh Barty in the semi-finals, dashing home hopes, while Muguruza outgunned an in-form Simona Halep, the reigning Wimbledon champion.

Along the way, Serena Williams' bid for a record-equalling 24th Major win fell flat in the third round, and 15-year-old Coco Gauff beat defending champion Naomi Osaka before going out to Kenin.

Moscow-born Kenin, 21, has so far lived in the shadows of her more illustrious compatriots but she will become the top-ranked American if she wins on her first appearance in a Grand Slam final.

Meanwhile Muguruza, after reuniting with coach Conchita Martinez, has rediscovered the form that deserted her in the second half of last season, when she only won one match in six tournaments.

The final, a rematch of their first-round match in Beijing in September, which Kenin won in three sets, promises plenty of aggression with both players thriving on taking the initiative.

"Of course I want to do the first punches. I don't want to be in defence against her," Kenin said.

"She can really put the ball away. She's really aggressive. So I feel like it's going to be who's controlling the points more, who is dictating."

'It's a lot about confidence'

Spain's Muguruza, who won the 2016 French Open and 2017 Wimbledon, arrived unseeded for the first time at a Grand Slam since 2014, and suffering from a virus to boot.

She promptly lost her first set 0-6 against qualifier Shelby Rogers but after recovering to win the next two sets 6-1, 6-0 she has not looked back, beating top-10 seeds Elina Svitolina and Kiki Bertens in consecutive rounds.

The 26-year-old said her returning confidence had been a key factor in her ability to come through tight matches like the semi-final, when she fought back from behind in both sets against Halep.

"I guess maybe structuring better the points, using more my weapons. It's literally a half-second or one shot the difference. It's very delicate," Muguruza said.

"It's also a lot about confidence, the way you're playing. Yeah, just a mix of that."

Muguruza barely celebrated reaching her first Australian Open final, indicating her eyes were already on a much bigger prize on Saturday -- when she will start as the favourite.

However, after a tournament that has made a mockery of predictions, she will be wary of one more plot twist on Rod Laver Arena.

"Definitely the mission is to get away from here with a big trophy," Muguruza said.

"But at the end the racquet has to speak out there. Doesn't matter how many Grand Slams you have, it's a tennis match," she added.

"Even if you have 15 Grand Slams, you go out there and you have somebody that can beat you. I'll take into consideration my previous experiences, but doesn't guarantee anything."