The former world number one overcame fellow Spaniard Carla Suarez Navarro 6-3 6-4 on Monday and was due to face seventh seed Kiki Bertens in the second round, but event organisers confirmed the Dutch player had been given a walkover to the last eight.
Kvitova overcomes Barty to take Sydney International title
With the draw for the Australian Open due to take place on Thursday ahead of the opening day of the year's first Grand Slam in Melbourne on Monday, 25-year-old Muguruza will be mindful of her physical condition.
She damaged a thigh muscle during the Hopman Cup in Perth last week, prompting her withdrawal from a mixed doubles match with France.
The former French Open and Wimbledon champion has dropped to 18 in the world over the past year, and is an 18-1 outsider with bookmakers for a third Grand Slam this month.
Her best performance at the Australian Open was a quarter-final appearance two years ago when she lost in straight sets to Coco Vandeweghe.
Halep stunned by Barty in Sydney second round
Kerber swats aside Giorgi to reach Sydney quarter-finals