Williams started strong and drove over Azarenka in the first set, but the Belarusian came back and defeated for the first time at eleven grand slam meetings Williams, who injured his ankle in the third set.
Thus, the chances for Williams to reach a 24th grand slam title in singles, and a touch of Margaret Court's record, are over for this time.
Latest final 2013
Azarenka is now ready for its third US Open final.
The last time she was in the final was seven years ago.
- Was it seven years ago?
That's my favorite number.
I guess that was the point.
I am very grateful for this opportunity, says Azarenka, who lost to Williams in the US Open finals in 2012 and 2013 - the same year she won double titles at the Australian Open.
Osaka champion 2018
In the final, Azarenka meets the Japanese Naomi Osaka, who earlier in the day won over the American Jennifer Brady with the set numbers 7-6, 3-6, 6-3.
Osaka won its first of two grand slam titles just over Williams at the US Open in Flushing, New York, two years ago.
- I really love the atmosphere even though there is unfortunately no audience here.
This course suits me really well, says Osaka.
The final is played the night before Sunday Swedish time.