Serena Williams has said that she "had to play well" in order to beat Dominika Cibulkova in the Australian Open quarter-finals.

Williams put in a dominant display as she dispatched the Russian in straight sets 6-2 6-2, but she insists that she wouldn't have won had she not been on top form.

"I feel I played well. I felt I had to. I feel like when you're going up against a player like that who is confident on the court - she just had a few good matches - I knew that I needed to really play well or go home," she told reporters.

Williams will play Madison Keys in the semi-final, after the young American knocked out Serena's sister, Venus, but Serena insists that she won't have it all her own way in the match, despite being the heavy favourite.

"I always root for Venus. At the same time, she's a very difficult player to play. She beat me the last time we played. It was a really long match. So I don't know. I mean, I can't say it's a relief because Madison has a big serve, a huge forehand. She's improved leaps and bounds. So it's going to be a tough match for me."

Williams and Keys are due to be third on court tomorrow.