“I was so mad,” Kostyuk said, smiling. “But actually when I’m mad, it’s helping.”

WTA rules dictate that 15-year-olds are limited to 10 tournaments, but her top-five finish in the junior rankings last year allows her to play in an additional two competitions.

There will be time for her to contemplate her future schedule, but for now there is a more pressing issue: She will face fourth-seeded Elina Svitolina, the best women’s player Ukraine has produced, on Friday in the third round.

Svitolina, 23, is coming off a title at the WTA tournament in Brisbane and would a prohibitive favorite under normal circumstances. Svitolina, however, said she has been struggling with unspecified “injury issues” before and during the tournament, and appeared unsure about her fitness.

“It was, like, big question that I’m going to step on court for my first round,” Svitolina said in a news conference after her match on Wednesday.

Although both are from Ukraine, the Zagreb-based Kostyuk and London-based Svitolina barely know each other. Svitolina said she heard of Kostyuk only after she won the junior title in Melbourne a year ago; Svitolina herself won the 2010 French Open girls’ title at 15.

“I will just enjoy it,” Kostyuk said of the match against Svitolina. “I think I’m going to play on big court again, but I will just try to show my best tennis, because I’m pretty sure, like, all my opponents, she will struggle a little bit, and I will try to use this.”

Although champions like Serena Williams, Martina Hingis and Steffi Graf won their first Grand Slam titles as teenagers, no player younger than 19 has captured a Grand Slam since Maria Sharapova won Wimbledon at 17 in 2004.