Azarenka rallied in New York on Thursday night for a 1-6 6-3 6-3 victory for the first major victory in 11 attempts against Williams.
Williams took a mid-game medical timeout for the Achilles tendon issue in the decisive second game and it was eventually broken 2-0. Unlike his previous three rounds, this time there is no comeback for the 38-year-old.
Williams has lost his last four Grand Slam finals as he tries to match Margaret Court’s record of 24, but a major semifinal defeat is rare. It dropped to 33-6.
Azarenka reached her first Grand Slam final after losing to Williams in the final of the 2013 US Open.
She will face Naomi Osaka, who defeated first-time Grand Slam semifinalist Jennifer Brady by a g-6 (1) -6-beat, -3–3. Osaka won the crown in 2018 by defeating Williams at the controversial end.
It is a rematch of the finals of the Warmup Western and Southern Open, which was also played on the site of the US Open as part of the tournament bubble during the coronavirus epidemic.
Osaka, however, withdrew from a hamstring injury to give Azarenka the title.
Thursday’s match was played with the roof at Arthur Ashe Stadium due to rain.