Coco Gauff put her hands on her face in disbelief, then pumped her arms in triumph on Sunday, celebrating the one thing her breakout season had been missing: a singles title.

Gauff, 15, won the Upper Austria Ladies Linz, an indoor WTA tournament, in the penultimate week of the tour’s regular season, beating the 2017 French Open champion, Jelena Ostapenko, 6-3, 1-6, 6-2.

The title made Gauff the youngest WTA singles champion in 15 years, since Nicole Vaidisova won two WTA titles in 2004, also as a 15-year-old.

“I’ll definitely remember this moment for the rest of my life,” Gauff said on the court as she held her trophy.