Maria Sharapova of Russia serves to Melanie Oudin of the United States during day six of the 2009 U.S. Open at the USTA Billie Jean King National Tennis Center in Flushing, Queens.
Azarenka, seeded eighth and ranked 18th in the world, will play the winner of a later match between No. 5-seeded Maria Sharapova and No. 3 Agnieszka Radwanska. She reached her third final of the season and is looking for her first title of the year.
Azarenka, who turned 21 on Saturday, has won 10 of her last 12 matches and is unbeaten in four career meetings against Stosur, who was playing her first tournament since reaching a career-high ranking of No. 5.