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.
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