Novak Djokovic has pledged financial support to Ukrainian tennis player Sergiy Stakhovsky, who has joined the fight to protect his country from the Russian invasion.

Stakhovsky, who famously beat Roger Federer at Wimbledon in 2013, last week said he had signed up for Ukraine's military reserves.

On Sunday the former world number 31 posted a screenshot on Instagram of a WhatsApp conversation with Djokovic.

"Thinking of you … hope all calms down soon," read a message from the Serbian 20-time grand slam champion.

"Please let me know what would be the best address to send help. Financial help, any other help as well."

Serbia has close political ties to Moscow. 

But last week the country voted in support of a United Nations resolution asking Russia to "immediately" withdraw from Ukraine.

Anti-war protests have also broken out throughout the country. 

The 36-year-old Stakhovsky retired after the Australian Open in January and has returned to Kyiv to take part in the struggle to repel the Russians.

A number of Ukrainian sports stars have joined the effort to defend their country, including two-time Olympic boxing gold medallist Vasiliy Lomachenko and world heavyweight champion Oleksandr Usyk.

"I have three kids. I have a wife and a place to stay outside of the Ukraine, so I cannot really justify exactly why I have done it, but home will always be where the heart is," Stakhovsky told the ABC last week.

"I am just trying to support the army in any way I can, and I just felt the responsibility and the duty to my country to stand our ground I guess."

Russian and Belarusian tennis players will be allowed to continue competing in ATP and WTA tournaments and grand slam events but they will not be allowed to compete under their countries' names or flags.

Team events and tournaments within Russia have been cancelled. 

AFP/ABC