Li Na of China hits a return to Anabel Medina Garrigues of Spain during their women's singles match at the Australian Open tennis tournament in Melbourne Jan 20, 2012.

MELBOURNE - World number six Li Na made into the fourth round at the Australian Open here on Friday after her opponent retired, when Nadal and Federer continued their winning run.

Li was leading 2-0 in the first set and on her serve in the third game when 26th-seeded Spaniard Anabel Medina Garrigues sprained her ankle after returning a ball. She then spent several minutes on the court being treated by medical staff.

Garrigues then walked from the court and had her ankle strapped before returning to play one more game, which she lost, before announcing retirement.

"The feeling is not good," said Li, considering this victory as unexpected, "because I didn't win the match by defeating my opponent."

"After two points (after she returned), I saw she couldn't run and she started to cry. I have to say sorry for her, because I knew the injury for tennis athlete is a bad thing. So I wish she was soon possible to come back to the court," said Li.

Li Na of China (L) checks on Anabel Medina Garrigues of Spain after she fell during their women's singles match at the Australian Open tennis tournament in Melbourne January 20, 2012. [Photo/Agencies]

Li's fourth round opponent will be defending champion Kim Clijsters of Belgium, a rematch of last year's final, won by the Belgian in three sets.

"I think we play the same way, like play against a mirror," said Li. "Only one day, so I couldn't change anything. Only thing is I just try my best on the court."

"Last year was the first time (for me) to be the finals. I didn't have any experience for the Grand Slam final. But after Australia, I think I did well in the French Open," added Li.

Li's compatriots Zheng Jie paired Polona Hercong to win in women's doubles second round while Peng Shuai lost in mixed doubles first round. Zheng Jie will play against 9th seed Frence Bartoli in Saturday's women's singles third round.

Roger Federer came through a tough test against big-serving giant Ivo Karlovic to join Grand Slam rival Rafael Nadal in the final 16 with a victory of 7-6 (8-6), 7-5, 6-3 in two hours 17 minutes. Nadal brushed aside his strongest challenge to defeat Slovakia's Lukas Lacko.

World No 1 Caroline Wozniacki continued her winning run with a commanding victory over Romanian Monica Niculescu while third seed Victoria Azarenka had a 6-2, 6-4 winner against Mona Barthel of Germany.

American John Isner's loss to Spaniard Feliciano Lopez saw no American players left in men's singles event, which created American tennis' worst record for 39 years.