Barty will play Caroline Garcia in the final after the Frenchwoman beat qualifier Maria Sakkari of Greece 6-3, 6-2 in the other semifinal.
Ostapenko, the last surviving seed at No. 8, appeared tired. Her previous three matches, including a win over top-ranked Garbine Muguruza on Thursday, all went three sets. But Barty’s three previous matches went the distance, too, plus she reached the semifinals in doubles.
Barty took command of their match with a break for 4-2. When she served out for the set, it began a run of seven straight games to finish the match and her third win over a top-10 player this week.
Unable to get going, Ostapenko complained to her coach and asked for treatment from a trainer. Neither helped the Latvian.
Garcia was hardly threatened by Sakkari, who was more tentative than she had been all week in her first WTA semifinal.
Garcia has reached her first final of the year, and will play for her fourth career title.
“It feels good,; it was long time without a final but a lot happened this year,” Garcia said.
She has never met Barty, who won her first career title in March in Kuala Lumpur.