Under bright sunshine but still unrelenting winds, world number one Sabalenka unleashed an ace out wide on match point to send fourth seed Rybakina packing from the year-end tournament.
"I didn't really think a lot about this match," Sabalenka told Tennis Channel. "I was just like, whatever. I'm going to go out there and do everything I can and if I win, I win and if I lose, I go home on vacation," she said with a laugh.
"I was just trying to play as good as I can."
Belarusian Sabalenka joins American group winner Jessica Pegula (29) in the semi-finals of the event between the world's top eight players. Kazakhstan's Rybakina and eighth-seeded Greek Maria Sakkari (28) were eliminated from the group stage.
In the other group, all four players - Iga Swiatek (22), Ons Jabeur (28), Coco Gauff (19) and Marketa Vondrousova (24) - are still in contention for the semis and due to play later on Friday.