Williams came through 6-4 2-6 6-4 this afternoon and will face Li Na in tomorrow's final.
The two women exchanged breaks midway through the first set after several punishing rallies. Williams then reached a drop-shot easily before a powerful forehand was too much for her opponent, and she took the opening set.
She was a different player in the second set though, although credit must go to Jankovic for her aggressive tactics. The Serb put Williams under pressure throughout the set and broke the American three times.
Williams raced into a 5-1 lead in the final set but allowed Jankovic back into it once more. The world number one held her nerve though, fending off break point in the final game with an ace followed by a whipped forehand winner down the line.