Katie Boulter wins her first WTA 500 title at the San Diego Open

The 27-year-old came back from a set down to beat sixth seed Kostyuk 5-7, 6-2, 6-2 in two hours and 13 minutes.

The victory marks Boulter’s second and biggest WTA title of her career after winning the Rothesay Open Nottingham trophy last summer.

Boulter has risen 22 places in the WTA rankings to a new career-high at world No.27.

"I don't know what just happened," Boulter admitted following her landmark final win.

"This week has been very, very special for so many different reasons. This one is pretty amazing, I've worked very hard for it, I played some incredible tennis all week.

"Today was a complete battle with myself as well because I was a little bit nervous. I managed to get over the line, and that I'm very proud of.

"A lot of it was about me staying as tough as I possibly could mentally, and I managed to keep my cool and actually kind of went within myself and calmed myself down a lot. I think that really helped me, and then I started to relax and play through shots a little bit more."