Sharapova, seeded No. 2, played for more than two hours in 85-degree sunshine. She won despite committing 52 unforced errors and losing her serve four times.
Sharapova is seeking her first Key Biscayne title after losing in the final three times.
Sharapova, who could face Serena in the semifinals and Venus in the final, said the return of the sisters is good for the women's tour.
"They've obviously achieved so much in tennis around the world and contribute so much to the sport," Sharapova said. "To have these two great athletes playing in your sport, it's always really good to see, not just for the fans, but for the game of tennis around the world."
On the men's side, No. 8-seeded Mardy Fish assured he'll remain the top-ranked American by beating No. 28 Kevin Anderson in the third round, 6-4, 6-3.