Paolini could have secured her ticket to next month's season finale in Riyadh with success on Saturday.
But the Italian's defeat means the battle for the last two spots will be settled at next week's Pan-Pacific Open in Tokyo, with Mirra Andreeva also still in contention.
"I knew the match was going to be very tough," said Rybakina, who improved her head-to-head record against Paolini to 3-3.
"Jasmine has played really well this season and she's a really tough opponent, so I knew I would need to bring my best.
"I'm happy I stayed focused until the end and won it in straight sets."
Former Wimbledon champion Rybakina entered the semi-finals having lost her last two meetings with Paolini, but utilised her big serve and powerful groundstrokes to win in 90 minutes.
Rybakina fired 30 winners, including 10 aces, and saved all seven break points she faced to reach her second final of the season, where she will face either Diana Shnaider or Ekaterina Alexandrova.