Serbian tennis star Novak Djokovic's son Stefan turned four recently and Djokovic and his wife Jelena took to social media to wish him.

Djokovic tweeted this throwback picture and wrote: "You have been a blessing to all since you arrived. I am so grateful to have you as my son, my brave falcon and knight You shine light & love on our souls. Thank you for teaching us how to be present, enjoy & smile, forgive & move on. Happy bday my dear son Love you."