The French Open runner-up, who had progressed to the last four with ease, was completely out of sorts as seventh seed Bovina wrapped up victory in only 56 minutes.
Dementieva, who reached her first grand slam final at the French Open in June before losing to compatriot Anastasia Myskina, had won three of her previous four meetings with Bovina, including one in Sydney at the start of this year.
On Friday, though, she struggled from the start and Bovina, 21, raced into her first final of 2004.
Bovina now plays eighth seed Nathalie Dechy after the Frenchwoman ended the run of unseeded American Lisa Raymond with a 6-4, 6-0 victory in the late match.
Dechy upset defending champion and third seed Jennifer Capriati in the quarter-finals on Thursday while Raymond beat Daniela Hantuchova of Slovakia.
