Djokovic, Wozniacki on Open path

Novak Djokovic's quest to upset Rafael Nadal and Roger Federer's Grand Slam stranglehold received a helping hand Friday as women's top seed Caroline Wozniacki stepped up a gear at the Australian Open.

Serbia's Djokovic, the 2008 champion, had taken the first set 6-2 when Davis Cup team-mate Viktor Troicki retired with a stomach strain, adding to a growing list of injury problems at the season's first major.

After Juan Martin del Potro's demise late Thursday, third seed Djokovic is the only player left in Melbourne, other than Nadal and Federer, to have won a Grand Slam since 2006. He said he was not sorry to play a short match.

"I think I had enough tennis already in the last couple weeks. It's good that you don't spend that much time on the court, maybe save some energy," Djokovic said. "But I'm going to be ready for the next match."

Djokovic will play Spanish 14th seed Nicolas Almagro in the round of 16 as he bids to shed the unwanted "one-Slam wonder" tag.

He is regarded as the chief threat to defending champion Federer and world number one Nadal, who is gunning to unite all four Grand Slam titles for the first time since 1969.

Meanwhile, Wozniacki took sweet revenge on Slovakian pocket rocket Dominika Cibulkova, whose shock victory at last week's Sydney International raised new questions about the Dane's credibility as world number one.

Wozniacki was broken three times but cashed in on an even higher rate by Cibulkova, the top 100's shortest player at five foot three inches (1.61 metres), wrapping it up 6-4, 6-3 in 1hr 32min.

"Dominika is a tough player -- I lost to her in Sydney last week so I knew it would be a tough match," Wozniacki said. "I'm just happy I could pull through."

French Open champion Francesca Schiavone eased past Monica Niculescu 6-0, 7-6 (7/2) and Latvian Anastasija Sevastova beat Vesna Manasieva 6-1, 6-3 to set up a last-16 meeting with Wozniacki.

Federer plays Xavier Malisse later while Chinese hope Li Na was facing Barbora Zahlavova Strycova and Japan's Kei Nishikori has a showdown with Spain's Fernando Verdasco.

Troicki joined a growing injury list after Marcos Daniel retired from his match with Nadal, Karol Beck was forced out while playing Andy Murray and a dizzy David Nalbandian gave up on his encounter with Richard Berankis.