Australian Open: Petra Kvitova crashes out to Madison Keys

Kvitova lost in straight sets, 6-4 7-5, to the young American, now being coached by Lindsey Davenport.

Eighth seed Venus Williams needed a set to get going before she beat Italy's Camila Giorgi 4-6 7-6(3) 6-1.

"That feels fantastic especially when things happen in your life that are not in your control," said the 34-year old, who struggles with Sjogren's syndrome, an auto-immune disease that can cause joint pain and fatigue. "But I don't want to stop now, I want to keep it going. This little cat has a few tricks up her sleeve."

And sixth seed Agnieszka Radwanska also continued her largely untroubled, yet under the radar, progress through the draw with a 6-0 7-5 win over Varvara Lepchenko and will meet Venus on Monday.

"I think playing her is always great challenge," Radwanska said. "I think she is still playing great tennis even she's a bit older. She's still really fit and playing a high level. I think it's going to be another good match."

Victoria Azarenka beat Barbora Zahlavova Strycova in straight sets, 6-4 6-4, and proceeded to swear in her courtside interview to describe her dismay at missing an attempted lob when serving for the match.

The 25-year-old told her interviewer the word could be "bleeped out" and later made no apologies.

"I'm having a lot more fun," Azarenka said. "I'm just being me. I say what I want to say. I laugh when I want to laugh. I play how I want to play. I grunt when I want to grunt."