Serbia's Ana Ivanovic pulled off the biggest upset so far at this year's Australian Open, defeating defending champion and top seed Serena Williams 4-6, 6-3, 6-3 in the round of 16 on Sunday.

"Going on to the court today, I really just tried to give my best. I tried to make it a good match. I was very competitive till the last moment, and I'm just very thrilled, obviously," Ivanovic said.

Williams, who has won the Australian Open five times, said after the match that she had been battling a back injury.

"Well, I guess (the) secret is out, but I obviously wasn't hitting the way I normally would hit and wasn't moving the way I normally would move and making a lot of errors that I normally would not make and I haven't made in a couple of years," the 32-year-old Williams, who is ranked No. 1 in the world, said in response to a question from a reporter about her back problems.

Ivanovic, who is ranked 14th in the world, ended Williams's 25-match winning streak and advanced to the quarterfinals at the year's first Grand Slam event.

"I just kept swinging. Even when I made errors, I really, you know, believed in it. So to have that victory, you know, it's amazing. We all know what kind of champion she is. When we were starting the match and they were talking about all her Grand Slam titles, it was quite impressive," the 26-year-old Ivanovic, who won the 2008 French Open, said.

The Serbian's opponent in the quarterfinals will be Canada's Eugenie Bouchard, who beat Australia's Casey Dellacqua 6-7 (5-7), 6-2, 6-0.

In other women's action, China's Li Na beat Russia's Ekaterina Makarova 6-2, 6-0, while Italy's Flavia Pennetta defeated Germany's Angelique Kerber 6-1, 4-6, 7-5.

There were no surprises on the men's side, with defending champ Novak Djokovic of Serbia, the No. 2 seed, beating Italy's Fabio Fognini 6-2, 6-0, 6-2.

Spain's David Ferrer, the No. 3 seed, defeated Germany's Florian Mayer 6-7 (5-7), 7-5, 6-2, 6-1, while the Czech Republic's Tomas Berdych beat South Africa's Kevin Anderson 6-2, 6-2, 6-3.

Switzerland's Stanislas Wawrinka knocked off Spain's Tommy Robredo 6-3, 7-6 (7-3), 7-6 (7-5). EFE