The latter's 6-3 7-5 victory ensured the world No.1 would be the first player to book a berth in the last four in Riyadh, Saudi Arabia.
Either Zheng or Paolini will join her but Kazakh Rybakina has been eliminated in the year-ending tournament for the top eight ranked players after successive losses.
Seventh seed Zheng bounced back from her first mach loss to Sabalenka to become the second Chinese player, after Li Na, to claim a victory at the Finals since their inception in 1972.
Zheng started strong, taking an early edge in the tiebreak. Rybakina rallied in the second set, using her serve to fend off break points and later securing a break of her own to level the match.
In the decisive set, Zheng punished a string of fatigued forehands from the Kazakh, sealing her victory after a break in the sixth game.
Rybakina suffered her second loss in Riyadh after entering the tournament on the back of fitness issues. Zheng, by contrast, entered it after winning titles in Palermo and Tokyo, and an Olympic gold in Paris.
"I'm really happy to win this match because I'd never beat her before and she's one of the greatest players right now on tour," Zheng said.
"Even though I had a chance in the second set and didn't take it, I'm happy I came back in the third set and stayed focused."
In the evening match Sabalenka cruised in the first set after building a 4-0 lead, then saved two set points in the second to complete her 22nd win in her last 23 matches.
Victory over Rybakina on Wednesday in the final group match will secure her the year-end No. 1 ranking for the first time in her career.
with AP
