By Alastair Himmer
Bosnian-born Petkovic celebrated reaching the last eight of a major for the first time with an on-court jig.
"It started off as a bet with my coach," Petkovic told reporters of the cheeky dance of delight she performed after her upset win.
"I'm very superstitious and since I started doing the dance I'm playing much better, so I'll try to keep it up," added the 23-year-old, who faces China's Li Na in the quarter-finals.
Squealing mightily on every shot, 14th seed Sharapova could do little right in the first set, spraying wild shots all over Rod Laver Arena.
The Russian showed a few signs of a possible fightback during the second set after going 2-0 down but the hole she had dug for herself was too big against a dangerous opponent.
She fended off two match points with a pair of rocket forehand returns but Petkovic completed victory in 79 minutes with a deep serve Sharapova could only push into the net.
"It definitely wasn't there," said a rueful Sharapova, who since winning here three years ago has reached just one grand slam quarter-final.
"Last year I was gone on Monday. I stayed here a little bit longer this year.
"But there are many things that didn't work today. I didn't get my first break point until it was a set and 5-1. That's not the type of tennis I want to be playing."
(Editing by Martyn Herman)