Serena Williams has said that she will have top be on top form to defeat Maria Kirlienko in the fourth round of the Australian Open.

The American has not yet dropped a set in Melbourne, but said that she must be "focused" against the 14th seed if she is to make the last eight.

"Maria [Kirilenko] is playing consistent tennis, especially the past 16, 18 months, she's been so consistent," said Williams. "So my goal is just to be really focused against such a player that's doing so well."

Williams was asked about a potential final against in-form Maria Sharapova, but stopped short of making any bold predictions.

"She's playing great," she added. "I think she's doing everything well.

"I have to get there. I have three more matches to win. I can't worry about that right now. I just have to take everything a step at a time."

The world number three has lifted the title in Melbourne on five previous occasions - the last coming in 2010.