Azarenka, a former world number one who has recaptured some of her best form at this year's tournament, recovered from a set down to beat Serena Williams 1-6, 6-3, 6-3 on Arthur Ashe Stadium in New York City.

Former US Open champion Osaka booked her place in the final with a 7-6 (7-1), 3-6, 6-3 victory over American Jennifer Brady.

Williams, seeking to equal Margaret Court's all-time record of 24 Grand Slam singles titles, looked in ominous form in the early stages and raced away with the first set in her clash with Azarenka.

But two-time Australian Open champion Azarenka, appearing in her first Grand Slam semi-final in seven years, rallied and levelled with a commanding performance in the second set.

Williams needed treatment for an Achilles problem in the third and was hampered physically for the remainder of the set, which Azarenka took to progress to Saturday's (September 12) final.

Azarenka has struggled on and off the court since returning after the birth of her son in 2016 and has even contemplated retirement, but said the win over Williams could provide inspiration for others.

"I needed to find energy to climb out of the hole and it wasn't easy," the 31-year-old said.

"Hopefully it inspires women to go after their dreams, they can do anything."

In the other semi-final, 2018 US Open winner Osaka also needed three sets to edge past 28th seed Brady.

The Japanese player won a pulsating first set on a tiebreak, before Brady hit back to send the contest into a decider.

But it was Osaka who prevailed in the third, emerging victorious from a high-quality encounter to progress to another Grand Slam final.

"The closer it gets to the finals, the more I think about - honestly, like no one remembers anyone but the winner," Osaka said. 

"Even if I do happen to not achieve my goal in the finals, at least I get a shiny little trophy. 

"At least I can leave with something."