Serena Williams has revealed that she played her fourth-round match at the Australian Open against Ana Ivanovic with a back problem.

However, the world number one, who lost the contest 4-6 6-3 6-3, has refused to blame her exit from Melbourne on injury.

"I almost pulled out," BBC Sport quotes Williams as saying. "I'm such a competitor, I probably should have.

"I don't want to blame anything. I feel like Ana deserves all the credit. I feel she played unbelievable today. I think she went for her shots."

"It's not anything that's life threatening or anything," she added on the injury. "I feel like it will get better. I just need a couple days off. Unfortunately I don't have that in a Grand Slam."

Ivanovic will take on either Eugenie Bouchard or Casey Dellacqua in the quarter-finals.