It will be the third straight final for the 23-year-old Serb, who won back-to-back titles at the China Open and the Porsche Grand Prix in Germany.
Jankovic is aiming for her fourth title this season and ninth of her career. She was runner-up two times this season.
Jankovic took the top ranking from Serena Williams on Monday and will retain it next week because Williams, the runner-up in Moscow last year, withdrew with an injury.
Dementieva, the Beijing Olympic champion, was playing in her 62nd career semifinal.
In the final, Jankovic will face the winner of the all-Russia semifinal between Vera Zvonareva and Dinara Safina.
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