Jankovic prevailed 6-4 7-5 in a tight match to set up a Sunday showdown with Germany's Angelique Kerber, who proved too strong for Australia's Sam Stosur.
Kerber beat the former U.S. Open champion 6-2 6-2 and is now one win away from securing her place in the eight-woman field at the WTA Finals in Singapore in eight days time.
"I'm not thinking too much about needing one more win tomorrow, though," the German said in an on-court interview.
"It's going to be another day and another match, and I'm just going to try and play some good tennis and win the final here."
Jankovic showed some good resilience in fighting back form 3-0 down in the second set to edge out the in-form Williams, winner of seven grand slam singles titles.
Williams needed to make the final to move into one of the eight qualifying slots for the WTA Finals.
"Playing against Venus is never an easy task. She's such a great champion and has been playing quite well the last month or so, winning Wuhan, so she's in great form," Jankovic said.
"I'm happy with the way I played, though, and I'm really happy I was able to beat her tonight."
(Writing by Patrick Johnston in Singapore; editing by Toby Davis)
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