The world No. 29 Dayana Yastremska was provisionally suspended for failing an out-of-competition doping test, the International Tennis Federation announced Thursday.

The ITF said Yastremska, a 20-year-old Ukrainian, tested positive for mesterolone metabolite, a prohibited substance that can be used to boost testosterone.

In a statement posted on her Twitter feed, Yastremska said she is “astonished and under shock.”

“I firmly state that I have never used any performance enhancing drugs or any prohibited substances,” Yastremska said.

A punishment would not be decided until after a full hearing. The ITF said Yastremska has a right to appeal the provisional ban but has not yet done so.

She reached a career-high ranking of No. 21 in January 2020, and her best showing at a Grand Slam tournament was reaching the fourth round at Wimbledon in her main-draw debut there in 2019.

Yastremska has won three WTA singles titles.

She said she passed a drug test at her most recent tournament, an event in Austria on Nov. 9.

“After this last tournament of the year, I stopped practicing to rest prior to the start of the new season,” Yastremska said.

She provided a urine sample on Nov. 24 that was tested at the World Anti-Doping Agency laboratory in Montreal, the ITF said.

“Only a very low concentration of mesterolone metabolite was detected in my urine. Given that low concentration, and given my negative test two weeks earlier, I have received scientific advice that the result is consistent with some form of contamination event," Yastremska said on Twitter. "Besides, I have been informed that this substance is meant for use as medication by men and that women are advised not to use it due to the adverse effects it causes.”

The 2021 tennis season started this week, including a WTA tournament in Abu Dhabi, United Arab Emirates. The year's first major championship, the Australian Open, is scheduled to begin on Feb. 8.

“I am currently working with my team and you can be assured that I am resolutely determined to do everything to clear my name,” Yastremska said.