Retired Ukrainian tennis player Sergiy Stakhovsky, 36, hopes he doesn't have to open fire while serving with Kyiv's volunteer defenders.
"I know how to use the gun. If I'll have to, I'll have to. I pretty much hope that I won't have to use the gun," the former world number 31, who famously beat Roger Federer at Wimbledon in 2013, told the BBC.
Top seed women's player Elina Svitolina made her feelings known by threatening to boycott a game against Russia's Anastasia Potapova in Monterrey this week.
In the end, she played and won the game, wearing a yellow top and blue skirt, the colours of the Ukrainian flag.
Several top Russian players have also come out against the war, including new men's world number one Daniil Medvedev, world number six Andrey Rublev and Russia's top female player Anastasia Pavlyuchenkova.
Ukrainian footballers led by English Premier League stars Oleksandr Zinchenko and Andriy Yarmolenko have also sought to rally opposition to the war.
"We ask all (the) world football community to oppose Russian propaganda, to show and tell the truth about war in Ukraine," said the Manchester City midfielder and West Ham winger in a video with 11 other top players.
The Ukrainian Football Association said an appeal by players had raised half a million euros ($556,649) for the army.
Russia has been barred from international football by FIFA and UEFA over the invasion, including the 2022 World Cup.
A young Ukrainian biathlete has already been killed while serving in the military, the International Biathlon Union said.
Yevhen Malyshev, 19, was a member of the national youth team from 2018 to 2020.
Dmytro Pidruchnyi, a world pursuit champion in 2019, is also preparing for combat.
In photos posted on social media, he says he has joined the national guard in the western Ukrainian city of Ternopil.
