Elina Svitolina said she will donate her prize money from WTA tournaments to Ukraine's military and to help with humanitarian efforts following Russia's invasion of her country last week.
The world number 15 told Eurosport her family and friends in Ukraine were defending the country and that she wanted to help.
"Really until the very last moment we did not believe that this war would actually start and then everything just happened at night ... Everyone is terrified, everyone is heartbroken," she said.
"My family is there. Lots of my friends who didn't leave the country are there. They're fighting for their life, some of them are fighting for our country. It takes a lot of courage and it's unbelievable that some people took weapons in their hands and went to fight for our land.
"The most painful thing I would say is I feel completely useless because I want to help them. I want to do something for them. Some of my friends are without electricity, without water, without food. They're really struggling."
Ukraine's health ministry said on Sunday that 352 civilians, including 14 children, had been killed since the beginning of Russia's invasion. It also said that 1,684 people, including 116 children, had been wounded.
Russia calls its actions in Ukraine a "special operation" that it says is not designed to occupy territory but to destroy its southern neighbour's military capabilities and capture what it regards as dangerous nationalists.
Svitolina, who is competing in Monterrey this week ahead of tournaments at Indian Wells and Miami in the United States, said she had endured a tough time mentally but acknowledged that what people were going through back home was worse.
"So I decided the prize money from my upcoming tournaments here in Mexico and in the States will go to the Ukrainian Army and to humanitarian needs," Svitolina said.
"So like this, I can help my country. And this, I think is the right thing to do at the moment. And I want to do something and to help my country."
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