An injury-hampered Keys defeated Venus Williams in three sets on Wednesday to continue her fairytale run at Melbourne Park and now faces the ultimate test against top seed Serena.

Williams has previously suffered defeat at the hands of a U.S. prodigy at Melbourne Park, beaten by teenager Sloane Stephens in the 2013 tournament, but has never been thrown off the top of the heap by year's end.

Into her first semi-final at Melbourne Park in five years after Wednesday's 6-2 6-2 rout of Dominika Cibulkova, Williams will be on her guard against Keys, long touted as the future standard-bearer for the women's game in America.

"She's playing great. I told her I was really happy that she did well," Williams told reporters of Keys. "She's in the semis. It's good to see another American, another African American, in the semi-finals playing so well. "Regardless, there's going to be an American in the finals, so that is great.

"It's also great for me and Venus because we know that finally there's other Americans that are constantly playing well and playing better, showing that they want to be the world's greatest.

"For sure I'll ask (Venus). (I will) also ask my coach, who has been steering me these last two weeks. I'm really excited to be in the semis."

Williams has complained of a cold in recent days at Melbourne Park and said it was getting "worse and worse", but there was little evidence of that as she broke her pattern of slow starts to rout last year's finalist Cibulkova in just over  an hour.

Breaking the Slovak 11th seed twice in each set, Williams gave nothing on serve and closed out the match when Cibulkova clubbed a return past the baseline.

The American grinned broadly as she smiled and waved at the Rod Laver Arena crowd on a sunny, breezy day. Keys will have American three-time grand slam champion Lindsay Davenport helping her prepare for the semi-final.

Williams beat old rival Davenport in three sets in the 2005 final, her second of five titles at Melbourne Park.

"I think Lindsay and Madison have done really well together. Obviously, this is her first semi-finals," she said. "I'm sure there's going to be many more, including grand slam wins, for Madison ... And I think they really just make a good team. Really, nice, easy-going people."

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