The biggest victory of her career thus far for the 27-year-old Briton came via a 5-7 6-2 6-2 victory.
Following a close opening set, Boulter defeated Ukrainian Kostyuk to win and move up to a career-high 27th position in the world rankings.
“I’m not sure what happened,” Boulter remarked following the game.
“It’s been a very special week for a variety of reasons. I’ve put a lot of effort into this one and played some terrific tennis all week. It’s fairly amazing.
