Iga Swiatek, who became the first Polish player to win a Grand Slam singles title when she beat Kenin in the French Open final, was named the WTA's Most Improved Player of the Year.
As Osaka walked off with a third Grand Slam title, Azarenka was denied a US Open crown for a third time having also finished runner-up in 2012 and 2013.
The Belarusian will meet Japan's Naomi Osaka in Saturday's final.
Azarenka overpowered Elise Mertens 6-1, 6-0, while Serena was forced to dig deep to secure a 4-6, 6-3, 6-2 quarterfinal victory over unseeded Bulgarian Tsvetana Pironkova.
Osaka's injury meant Belarusian Azarenka won her first title since 2016.
Osaka beat Belgian Elise Mertens to reach the Western & Southern Open final after playing in a semifinal she had originally withdrawn from in a call for racial justice.
Halep and Azarenka were among four former WTA number ones in singles action on day three.
Two-time Grand Slam winner Azarenka, who won the Cincinnati Open in 2013, will return to the women's draw for the first time since 2016.