Top Ukrainian junior Yelyzaveta Kotliar said she was "deeply sorry" after her tennis federation criticised the 16-year-old for shaking the hand of a Russian opponent at the Australian Open.
Kotliar came under scrutiny at the opening Grand Slam of the year when she congratulated Vlada Mincheva after being beaten in the first round of the girls' tournament.
The handshake defied an unwritten rule among her fellow Ukrainians to not congratulate their Russian and Belarusian opponents as war rages in their homeland.
The Ukraine Tennis Federation called it an "unpleasant incident", reiterating its position that "Ukrainian tennis players not hold handshakes with representatives of aggressor countries".
"Our leading players have been informed about this and support this position," it added in a statement, calling Kotliar's action "a mistake".
Kotliar's father Konstantin Kotliar said his daughter was not thinking clearly in a pressure situation.
"She is only 16 years old and has no real experience of appearing in major competitions like Grand Slam tournaments, the pinnacle of both professional and youth tennis," he said in statement on her behalf.
"She performed a post-match ritual, shook the hand of the opponent, did not understand that behind the net was a representative of the country who attacked our homeland.
"It was definitely a mistake that she is deeply sorry (for) and assures that she will never allow anything like this again."
Earlier in the tournament, Ukraine's Lesia Tsurenko, the 28th seed in the women's main draw, said the handshake snub had become part of the players' protest against Russia.
"This is another reminder that there is a war in my country. If you can just imagine (being) in my place or any other Ukrainian you would not have this question," she said.
"This is the right thing to do and I do this for Ukraine."