A former quarter-finalist at Melbourne Park, Kvitova missed last year's event as she recovered from surgery to her left hand which was injured by a knife-wielding assailant weeks before the tournament.
Kvitova showed she was back to a level approaching her dominant best with a last-eight appearance at the U.S. Open, but her clash with former top-10 player Petkovic ended in heartbreak after she double-faulted on match point when serving at 5-4 in the third set on Show Court Two.
Petkovic saved another match point with a forehand winner to level at 5-5 and fought back from another break down.
Serving to stay in the contest, 27th seed Kvitova suffered an attack of nerves, missing a straightforward volley before surrendering the match with her 10th double-fault and sixth of the set.
The crowd groaned in disappointment, while Petkovic roared in triumph and Czech Kvitova walked off to warm applause in the twilight.
"I just thought about not thinking and it worked," Petkovic said of holding on during the nervous finish.
(Editing by Ed Osmond)
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