Serena Williams produced a relentless and brutal display as she reached the Australian Open semi-finals on Wednesday and the top-ranked American said she had no other choice against Dominika Cibulkova.

Williams dispatched the diminutive Slovakian in a little over an hour on Rod Laver Arena, easing to a 6-2 6-2 victory.

The 18-time grand slam champion hardly gave Cibulkova a sniff in their quarter-final clash, tallying 15 aces and 31 winners.

Williams also did not face a break point as she booked a semi-final showdown against American teenager Madison Keys.

"I feel I played well. I felt I had to," the 33-year-old told a media conference.

"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."

While Williams made light work of the 25-year-old, it does not appear to be all smooth sailing for the veteran.

Williams revealed post-match that she is suffering from an illness, which is showing no signs of subsiding ahead of the semis.

"I've been sick the past few days," she added.

"It's just getting worse and worse. But hopefully it will start getting better."

Cibulkova, who lost to Li Na in last year's final, offered an easy explanation for her defeat on Wednesday " Williams was just too good.

The Slovakian 11th seed was limited to just 38 points throughout the contest, while Williams won 58 of her own.

"She was just playing really well today, I have to say," Cibulkova said.

"She was putting so much pressure from the serve and return. I didn't have a chance to play my game.

"Just felt under so much pressure. It was a good day for her."