A 10th career title for Rybakina means the Kazakh only has to reach the semi-finals of the Pan Pacific Open in Tokyo this week to seal the last qualifying spot for the season finale at the expense of Russian teenager Mirra Andreeva.
Others who have qualified for the WTA Finals, to be held in the Saudi Arabian capital of Riyadh from November 1-8, are Aryna Sabalenka, Iga Swiatek, Coco Gauff, Amanda Anisimova, Jessica Pegula, Madison Keys and Jasmine Paolini.
"I'd like to congratulate Ekaterina for a great week and great season," said Rybakina.
"It hasn't been an easy year for all of us, a difficult schedule ... thank you so much to my team for always pushing me to be better.
"The end of the season isn't easy but without you I wouldn't be here. Hopefully, we can push a little more this week (in Tokyo)."
Alexandrova made a flying start, racing 4-1 ahead as Rybakina struggled to rein in the errors and mix up her game, before the 30-year-old comfortably took the opening set with a powerful forehand winner.
Desperate to prevent a fourth straight defeat by Alexandrova on hardcourts, Russian-born Rybakina came out firing in the next set, hitting a crosscourt winner to consolidate an early break and lay the platform to level in the match.
World number nine Rybakina cranked up the intensity from the baseline in the deciding set, but it was a foray to the net that earned the 26-year-old another early break, and she went on to claim her second trophy of the season.