The 20-year-old, runner-up at Melbourne Park last year, overpowered 11th-seeded Dementieva, breaking serve six times on her way to victory in 62 minutes.
"I have always had really tough encounters against her and I wanted to make sure I kept my focus all the way through," Sharapova said in a courtside interview.
Sharapova was in scintillating form and gave Dementieva no chance to settle, easing through the first set in 32 minutes.
The former world number one was even better in the second set, rushing through it and crunching a huge forehand return to reach the last eight for the fourth successive year.
"I am really looking forward to my next match against Justine," she said.
"We had a really tough one in Madrid at the end of the year, that she just pulled through. She's a great player.".