"I came here with every intention of making a strong start to the year, but sadly my team and I don't feel that I'm where I need to be just yet to compete at the highest level".
"I feel I need to play matches to see exactly where it's at".
"I'm aware of the situation at the end of previous year (2017) especially because for tennis and the sport it's not good", Murray said of the injury situation. I'm just hoping I can get back to a level where I'm able to be really competitive.
The Scot - a two-time champion in Brisbane - did play one set against Roberto Bautista Agut at the Mubadala World Tennis Championship on Friday, losing 6-2.
"I don't really cramp a lot, so I'm actually surprised that today my body was like that", Muguruza said, adding that her calf muscles started to cramp early in the second set.
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After his 6-7 (5-7) 7-6 (7-4) 6-4 win, the Brit will take on South Korea's Hyeon Chung.
Murray, who is in Queensland for the season-opening Brisbane International, has not played on the ATP tour since losing a tough five-setter to American Sam Querrey in the quarter-finals at Wimbledon.
The Brisbane International had been Muguruza's only scheduled tuneup event before the first major of the season gets under way in Melbourne on January 15.
Murray admitted he wasn't heading into this tournament or the Australian Open with great expectations.
Fellow "Big Four" members Novak Djokovic and Rafael Nadal remain doubtful to compete at Melbourne Park, while 2014 champion Stan Wawrinka is also set to be absent. "I'm hoping that that doesn't continue happening".