Sabalenka triumphed 6-2 3-6 6-3 on Friday, in a round-robin match repeatedly interrupted by rain the night before until being suspended late in the second set.
Play was halted Thursday night with Rybakina about to try to force a third set by serving for the second at 5-3.
When they resumed she did just that, but Sabalenka came through in the decider although not without a bit of tension in the last game.
She needed to save five break points and then required four match points to end it.
This was a rematch of the Australian Open final in January, also won by Sabalenka.
Sabalenka went 2-1 in round-robin play at the season-ending championship and advanced as the second-place finisher in her group. Jessica Pegula, who beat Maria Sakkari on Thursday to go 3-0, finished atop their group.
Rybakina was eliminated with Friday's loss.
There were a half-dozen rain delays during Thursday's stop-start-stop action.
The two semi-finalists from the other singles group were scheduled for later Friday, with US Open champion Coco Gauff taking on Wimbledon champion Marketa Vondrousova, and four-time major champ Iga Swiatek meeting three-time slam runner-up Ons Jabeur.
Earlier, the Women's Tennis Association conceded its tour Finals in Cancun was "not a perfect event" and signalled changes after a series of complaints from top players about how they were treated.
In a letter to the players, leaked to Sports Illustrated, WTA chief Steve Simon said the late selection of the Mexican resort to host the WTA's showpiece end-of-season tournament was "based on a number of complicated factors".
"It is clear you are not happy with the decision to be here in Cancun. I understand that and you have been heard," he wrote after thanking the players for attending a meeting in Cancun.
"It is not a perfect event, we understand the conditions are a challenge and the WTA accepts responsibility for that."
Cancun was named as venue for the $US9 million ($A14 million) tournament, which brings together the top eight singles players and doubles partnerships to conclude the season, less than two months before it started on October 29.
World No.1 Sabalenka had blasted the WTA over the court conditions after the opening day of group play and said she felt "disrespected" by the standard of organisation at the prestigious event.
with AP
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