Serena Williams Powers Into Australian Open Semi-Finals

A fired-up Serena Williams shot down last year's finalist Dominika Cibulkova Wednesday to set up an all-American Australian Open semi-final with teenager Madison Keys, who brought sister Venus' dream run to a grinding halt.

Unlike her earlier sluggish matches, Serena was in the zone from the off against the dangerous Slovak, a steely look in her eyes as she walked onto Rod Laver Arena soon after Venus lost to Keys.

The powerful world number one blasted 15 aces to pummel the 11th seed 6-2, 6-2 in just over an hour and make her 26th Grand Slam semi-final. She has gone on to win 18 major titles.

"I knew that I had to beat her today because she was in the final and she plays really well on this court so I'm really happy to go through," said Serena, who has been battling a cold and could barely talk.

Long-time rival Maria Sharapova made the last four on Tuesday and will face fellow Russian Ekaterina Makarova.

The unseeded Keys, 19, is the tournament surprise package, battling hard to beat a resurgent Venus 6-3, 4-6, 6-4 and end any chance of the sisters meeting in a Grand Slam semi for the first time since Wimbledon 2000.

Keys did the job despite carrying a left thigh injury, needing a medical timeout in the second set for treatment.

"I think just being able to come back from being down and from not being able to move as well ... just being able to kind of grind through that, still manage to win some points, is what I'm most happy about," she said.

Among the men, Andy Murray and Tomas Berdych are already safely into a semi-final showdown after beating Nick Kyrgios and Rafael Nadal respectively on Tuesday.

Aiming to join them is world number one Novak Djokovic, who is gunning for a fifth Australian title and meets the challenge of big-serving Canadian Milos Raonic later Wednesday.

The Serbian top seed has a 4-0 record against Raonic and has been in fine touch.

The winner will face either Swiss defending champion Stan Wawrinka or fifth seeded Japanese superstar Kei Nishikori.

- Ominous sign -

In an ominous sigh for Keys, Serena, chasing her sixth Australian title, has made the semis in Melbourne five times before and gone on win the championship on each occasion.

But Keys, now coached by former Australian champion Lindsay Davenport, is undaunted.

"It's just one of those things where I have to go out and I have to do my best and I have to really just stay focused on my side of the court," she said.

"Because she's obviously very, very good and she's going to play very well."

Keys is seen as one of the new generation to take up the mantle from the Williams sisters and Venus was generous in her praise.

"I give a lot of credit to her because she really set her points up. She was swinging freely. Most of them went in for her," said the 34-year-old, a childhood idol of Keys.

"So it was just, you know, great for her."

Despite the defeat, Venus is expected to move as high as 11 in the world rankings following her best run at a Grand Slam since 2010 coupled with winning the lead-up Auckland Classic.

She said it was an encouraging start to her year.

"It's definitely been a great last four weeks. A lot of match wins, a lot of momentum, just a lot of great things happening that everybody wants to happen in their career."