Azarenka, 24, from Belarus, didn't play error-free tennis. But against an opponent who had never been this far in a major tournament, Azarenka didn't have to be at her best.
Azarenka beat unseeded 31-year-old Flavia Pennetta 6-3, 6-2, in one of the women's semifinal matches.
There could be a rematch of last year's women's final, in which Serena Williams came back to win the last four games of the final set for the victory. Top-seeded Williams is facing fifth-seeded Li Na, the first Chinese player in the semifinals, in the second singles match.
Azarenka was the youngest semifinalist; the other three are 31. Azarenka has won two straight Australian Opens and is 25-7 at U.S. Opens.
Williams is still alive for her fifth U.S. Open while Azarenka has only reached the finals.
Pennetta hadn't lost a set in the tournament before Friday. Azarenka had spent more time on the court than any of the semifinalists -- eight hours, 19 minutes -- an indication she must be saving her best tennis for last.
Pennetta had become the first Italian woman ever to be ranked in the top 20 in 2009 but she had wrist surgery that caused her to miss the 2012 U.S. Open and came here ranked 83rd in the world.
So far this season Azarenka is 31-1 on hardcourts, with the only loss coming to Samantha Stosur in Carlsbad.