Novak Djokovic has offered financial support and any other help required to Ukrainian former world number 31 Sergiy Stakhovsky, who has enlisted in his country's reserve army amid Russia's invasion of its neighbour.

Stakhovsky, who won four ATP titles and had a shock win over Roger Federer in the second round of Wimbledon in 2013, joined Ukraine's reserve army last month despite a lack of military experience.

The 36-year-old posted a screenshot on Instagram of his conversation with Djokovic via WhatsApp, in which the Serbian world number two asked about his situation and hoped for things to calm down.

"Please let me know what would be the best address to send help ... financial help, any other help as well," Djokovic said in the message to the retired Stakhovsky.

Stakhovsky posted on Instagram that Ukraine was grateful for Djokovic's offer of support.

Russia calls its actions in Ukraine a "special operation" designed not to occupy territory but to destroy its neighbour's military capabilities and capture what it regards as dangerous nationalists.

Several other Ukrainian athletes, including world boxing heavyweight champions Wladimir Klitschko and Vitali Klitschko, are defending their country after taking up arms.