Easy wins for Sharapova, Ferrer

Fourth seed David Ferrer used all of his cunning and exploited Kei Nishikori's nagging knee injury to advance to the quarter-finals of the Australian Open with a 6-2, 6-1, 6-4 victory on Sunday.

Japan's Nishikori, who had entered the tournament with a knee injury, needed to have his left leg re-strapped in the second set of the two hour, 10-minute match, and his normally fluid movement appeared to be slightly hampered.

Baseline specialist Ferrer took full advantage of the 16th seed's injury, working him around the court and waiting for the 23-year-old to make one of his 65 errors, though the Japanese number one did put up some resistance in the third set.

The Spaniard will next play the winner of the fourth round encounter between eighth-seeded Serbian Janko Tipsarevic or 10th seeded compatriot Nicolas Almagro in the quarter-finals.

Ekaterina Makarova sprung a major upset at the Australian Open for the second successive year on Sunday when the Russian completed a 7-5, 6-4 victory over fifth seed Angelique Kerber in their fourth round encounter.

The 19th seed started brightly and survived a wobble to close out the first set before going on to defeat her injury-hampered German opponent, the highest seed so far to lose in the women's draw, in one hour and 32 minutes with an ace.

Makarova advances to her second consecutive quarter-final at Melbourne Park after matching her run at last year's tournament when she sensationally defeated five-time champion Serena Williams.

Makarova will meet the winner of the clash between second seeded compatriot Maria Sharapova, who ended her run 12 months ago, and unseeded Belgian Kirsten Flipkens.

Maria Sharapova took a little bit of time to warm up, but continued her progress through the Australian Open with a 6-1, 6-0 victory over Belgium's Kirsten Flipkens on Sunday.

Sharapova had been the most dominant player in the women's draw so far, recording successive 6-0 6-0 victories in the first two rounds before she dispatched seven times grand slam winner Venus Williams with contemptuous ease in the third round.

The second seed, however, took time to get into her match against the 27-year-old Flipkens and while she won the first set 6-1, it took 43 minutes which demonstrated how difficult she found the tenacious Belgian to put away.

Sharapova, however, cranked into gear in the second set to clinch it in 25 minutes and set up a quarter-final against fellow Russian Ekaterina Makarova after the 19th-seed dispatched fifth seed Angelique Kerber 7-5, 6-4.

Nicolas Almagro will meet fellow Spaniard David Ferrer in the Australian Open quarter-finals after his fourth-round opponent Janko Tipsarevic retired hurt in their match on Sunday.

The eighth seeded Serb Tipsarevic received treatment on a foot injury and pulled the pin when trailing 6-2, 5-1 in the match at Hisense Arena.

The 10th-seeded Almagro advances to his first quarter-final at Melbourne Park in his ninth appearance at the year's first grand slam after making the last eight at the French Open three times.

Tipsarevic had played two successive five-set matches coming into the Almagro clash and said in the lead up that his legs were going to "feel" it.

Agnieszka Radwanska continued her perfect start to the year, defeating former world number one Ana Ivanovic 6-2, 6-4 to march into the quarter-finals of the Australian Open on Sunday.

The 23-year-old made just four unforced errors to 34 for her Serbian opponent and saved five break points in the second set before sealing victory with her seventh ace after 70 minutes.

Radwanska won the titles at both her warm-up tournaments in Auckland and Sydney and has not dropped a set in 13 straight victories this year.

The Pole will next face China's sixth seed Li Na, who she beat in the semi-finals at the Sydney International 10 days ago.

Defending champion Novak Djokovic survived an early onslaught and a late fightback from Stanislas Wawrinka in a pulsating fourth-round clash to advance to the quarter-finals of the Australian Open early on Monday.

The 15th-seeded Swiss had taken an early 6-1 5-2 lead over the world number one with a powerful serve and booming groundstrokes before Djokovic took greater control of the rallies and waited for Wawrinka to make mistakes.

Wawrinka, however, managed to send the match into a decider when he totally dominated the fourth set tiebreak before the world number one clinched a 1-6, 7-5, 6-4, 6-7, 12-10 victory on his third match point with a cross-court backhand winner.

The match, which lasted five hours and two minutes.

"It's really hard to find the words to describe the feeling," an exhausted Djokovic said courtside.

"He deserved equally to be a winner of this match. I give him a lot of credit. He showed his qualities.

"He was the aggressive player on the court. He was playing well, mixing it up, using the right tactics and serving really well.

"I was just trying to hang in there and fight and give every drop of energy."

Djokovic, seeking to become the first man in the professional era to win three successive Australian Open titles, will now meet fifth seed Tomas Berdych in the quarter-finals.