The 22-year-old Spaniard had been considered one of the contenders for the title but looked lethargic and unable to establish a rhythm against the Czech, who capitalised on her excellent tactical game to win the match in just 76 minutes.
“Barbora was making me move. She was being smart,” Muguruza told reporters. “I felt that I was not moving very well. I guess also I didn’t feel quite good at any moment.
“I think I just couldn’t find the court, my shots and didn’t really find my game.”
Strycova, who had reached a career-high of 20 last year before her results and ranking fell away, had entered the season-opening grand slam ranked 48th, though her standard of play belied that.
She kept the Spaniard off balance and capitalised on a service game directed close to Muguruza’s body that restricted her ability to swing from the hip on return.
“It’s not easy against Garbine, because she plays fast and also very deep,” Strycova said.
“I felt good on court. I was serving very well and the plan was like to play my game, mix it a little bit, but to play deep.
“That’s why I think I won today.”
It is the first time the 29-year-old Strycova has made the fourth round at the Australian Open and will now play twice champion Victoria Azarenka in the last-16.
“Victoria is very, very good. I saw her hitting some balls yesterday and she’s hitting the ball very well,” Strycova said of the Belarusian before adding that comfortably accounting for Muguruza had lifted her confidence.
“I lost against Garbine in Tokyo pretty easy and I won today. So I will go into that match with confidence.
“But I will have to move well, serve well. Against these players you have to play deep balls and try to mix it.
“I have to play my best tennis.”
(Reporting by Greg Stutchbury in Melbourne; Editing by Peter Rutherford/Patrick Johnston)