Serena Williams broke her run of slow starts at the Australian Open on Wednesday to hammer Dominika Cibulkova 6-2 6-2 and set up an all-American semi-final against teenager Madison Keys.

An injury-hampered Keys dug deep to beat Williams' older sister Venus in three sets earlier in the day, but Serena was in no mood for a contest as she broke the Slovakian 11th seed twice in each set and gave her nothing on serve.

Williams closed out the match when last year's finalist Cibulkova clubbed a return past the baseline and the American grinned broadly as she smiled and waved at the Rod Laver Arena crowd on a sunny, breezy day.

Top seed Williams will head into the semi-final fresh after her one-hour five-minute rout as she bids to win her first title at Melbourne Park since 2010.

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