The 30th-seeded Petkovic of Germany was rarely challenged in beating the 2008 Australian Open champion 6-2, 6-3 on Sunday. Petkovic benefited from an easy third round — she only needed to play seven points before seven-time major winner Venus Williams retired on Thursday night with an injured hip muscle.
She will now take on China's Li Na in the quarterfinals.
Meanwhile, Caroline Wozniacki moved into the Australian Open quarterfinals with a 6-3, 6-4 win over Anastasija Sevastova on Sunday and is just one victory from ensuring she'll retain the top ranking.
The 20-year-old Dane, playing her first major as world No. 1, reached the quarterfinals at Melbourne Park for the first time. She can keep her top ranking with an appearance in the semis.
She wants more than that, though. She's aiming for a first Grand Slam title.
The questions about Wozniacki's worthiness of the top ranking are starting to diminish. Asked which player she feared most, she said: "I don't fear anyone actually."
"I'm feeling confident. I feel like I can beat anyone on a good day. I think they have to fear me when they're playing me," she said. "I just go out there and do my best. If they're better than me that day, it's just too good."
Wozniacki lost three of the first four games of the match before reeling off six in a row, including a break in the first game of the second set. Sevastova, ranked No. 46, rallied and the pair traded breaks until Wozniacki regained control.
From Margaret Smith Court to Serena Williams, check out the Open era victors.
more photos
But she later tweeted: "Round 2 with the media:) hope you enjoyed my kangaroo story, hope you know i was just kidding:) see you on tuesday for round 3!"She later returned to Melbourne Park to clarify she'd made up the story and to apologize, saying she didn't think anyone would believe it.
French Open champion Francesca Schiavone established a Grand Slam women's record later Sunday, needing 4 hours, 44 minutes to beat Svetlana Kuznetsova 6-4, 1-6, 16-14 in a fourth-round match. Schiavone saved six match points, then converted on her third match point in the longest match at a major in terms of time in the Open era. The longest previous women's match in a Grand Slam tournament was here last year when Barbora Zahlavova-Strycova beat Regina Kulikova 7-6 (5), 6-7 (10), 6-3 in a match lasting 4:19.
Schiavone will meet top-ranked Wozniacki in the quarterfinals.
Li Na, already China's most successful player, reached the quarterfinals with a 6-3, 6-3 win over eighth-seeded Victoria Azarenka of Belarus.
The ninth-seeded Li was one of two Chinese players to reach the semifinals at Melbourne Park last year, her run ending with a loss to eventual champion Serena Williams in two tiebreak sets.
There's more Chinese players into the fourth round than from any of the four Grand Slam host nations.
Nadal, Wozniacki win ITF World Champion awards
