Dayana Oleksandrívna Yastremska (Ukrainian: Даяна Олександрівна Ястремська; born 15 May 2000) is a Ukrainian professional tennis player. She has a career-high Women’s Tennis Association (WTA) ranking of No. 21 in the world. Yastremska has won three WTA titles in four finals appearances. A junior Grand Slam runner-up in both singles and doubles, Yastremska had a quick breakthrough onto the WTA Tour. She made her debut in the top 100 and won two titles when she was 18 years old, including her first at the Hong Kong Tennis Open in 2018. She had a successful 2019 that helped her rise from No. 58 at the start of the year up to No. 22 by the end of the season. Yastremska was suspended from competition at the start of 2021 after testing positive for mesterolone. On 22 June 2021, the International Tennis Federation ruled that Yastremska was not responsible for the positive result, and that she is eligible to return to competition immediately. She made her return to the tour at the Hamburg Open.
Martincová has won four singles titles on the ITF Circuit. On 1 November 2021, she reached her best singles ranking of world No. 48. On 27 September 2021, she peaked at No. 188 in the WTA doubles rankings. In June 2013, Martincová made her WTA Tour main-draw debut at the Nürnberger Versicherungscup. After coming through all three qualifying rounds, she lost to Estrella Cabeza Candela in round one. One month later, Martincová once again came through qualifying at the Baku Cup, defeating Oksana Kalashnikova in the first round, but losing to Tadeja Majerič in the second.