The top-seeded Williams outlasted Jankovic 6-4, 2-6, 6-4 and advanced to Sunday's finals where she will take on Li Na of China.
Li cruised past fifth-seeded Petra Kvitova 6-4, 6-2 Saturday to reach the finals.
Williams, who is 9-1 lifetime against Li, won the first set but Jankovic took advantage of a spate of errors by Williams to win the second set. The fatigued American rallied in the third set and won her fourth match point before closing the match out.
Couple calls 9-1-1 over missing hash browns; assault McDonanld's employees
Britney Spears debuts 'Perfume' video