Serena Williams has admitted that the form of her sister is driving her on to succeed after booking her place in the third round of the Australian Open.

Venus Williams continued her unbeaten start to 2015 with a win over Lauren Davis on Thursday, while Serena proved to be too strong for Vera Zvonareva in her own second-round match.

"It like makes me look behind my shoulders, and I have to play better and I want to do better. I always want to be able to stay ahead as much as I can. For our whole careers, we've kind of motivated each other. We hopefully continue to do that," the top seed told reporters.

"She's done so much in her career. She doesn't have to win another match. The same thing for me. As long as we can kind of look at it that way, then we both will do really well."

Serena will meet Elina Svitolina in the third round, while Venus comes up against Camila Giorgi on Saturday.