The world number 23 beat eighth-ranked Mirra Andreeva 6-3, 7-5 in under 90 minutes on Saturday to secure the most prestigious trophy of her career, her third professional singles title and second in less than a month after Rouen.
Yet as the 23-year-old Ukrainian posed for photographs at the Caja Magica, it was not just the silverware that caught the eye.
Held alongside her team and her two dogs, Kostyuk showed off a piece of black men's underwear, prompting raised eyebrows and plenty of laughter.
"We have a tournament laundry service, and sometimes the clothes get mixed up, and last year we ended up with this piece of underwear where in it was written 'Lucky Shorts'. So we kept it," Kostyuk told Spanish broadcaster Movistar Plus.
"My coach Sandra (Zaniewska) kept it as a lucky charm.
"She drove to her father's house just to pick it up before travelling here again for the tournament. And we made this promise that if I'd win we would bring the 'Lucky Shorts' for the photoshoot with the trophy."
Kostyuk was delighted to finally conquer a tournament she had long admired.
"This trophy tastes like champagne, no doubt about it," she said.
"Honestly, watching this tournament as a child and remembering all the people who've won it before me, I never thought this was possible, it's amazing.
"It's one of the toughest tournaments of the year, and everyone plays so well, especially now. So it certainly tastes good, but I just want to enjoy this today and that's it."