The 25-year-old Russian, who won the Melbourne Grand Slam in 2008, has lost just four games during her first three matches at this year's tournament.
"The reason I started so well in this tournament is because I knew I had to," said Sharapova. "It's tough. I hadn't played a professional match, a real match, in over two months.
"Coming into a Grand Slam it's not always the best-case scenario, but I was coming in knowing I was healthy and ready to go. I was much more mentally prepared."
Sharapova will face Kirsten Flipkens on Sunday for a place in the quarter-finals.