The Spaniard couldn't match the physicality and fearlessness of young American Sofia Kenin, losing their Saturday night decider on Rod Laver Arena 4-6 6-2 6-2.

Two-time major champion Muguruza's trekked to the top of Mount Kilimanjaro in the off-season but she hadn't put together seven grand slam matches since 2017 when she won the Wimbledon title and said she found the going tough.

In a fairly lean two-and-a-half years since, the former world No.1's ranking had dropped to 32 while she'd only made it past the fourth round at a grand slam once.

"I did feel I lacked a little bit of energy after so many matches and physically it was a tough battle out there," Muguruza said.

"My body was feeling the two weeks, that's for sure."

"Also this match was a lot of rallies, a lot of running - at some point I was feeling my body."

With the French Open and Wimbledon crowns already in her keeping, the 26-year-old was looking to add the Australian Open to her collection.

But it was a case of missed chances for Muguruza, converting just two of 12 break-point opportunities.

The Spaniard had treatment on her back after dropping the second set but she said it wasn't a new injury.

Nonetheless, her serve went off the boil in the decider.

At 2-2 Muguruza had three break points but Kenin blasted four winners to take the game and crush the Spaniard's spirit.

She finished with eight double-faults including one on match point to hand world No.15 American the title.

"It was a very important moment of the match," Muguruza said of the fifth game in the third.

"I think she found very, very good shots during the match, especially in the important moments she came out with winners.

"I'm not very happy about my performance - I think I had to play better because she came up with a great level."

While disappointed with the defeat, Muguruza felt positive about the year ahead.

Unseeded at a slam for the first time in almost six years, she will halve her ranking and rocket to 16 next week on the back of her impressive run.

"I think I got to keep working physically as these events are really long, so you got to be strong," she said.

"I think it's a great start of the year, for sure. It's going to give me a lot of information about where I am right now."

Australian Associated Press