Novak Djokovic was in high spirits ahead of his quarter-final match at Wimbledon on Monday.MORE: Game Set Match: The Wimbledon wives and partners on courtside this summerThe Serbian tennis star was seen messing around with his son Stefan during practice – and the pair seemed as happy as can be! WATCH: Kate Middleton makes surprise appearance at WimbledonStefan, seven, could be seen clambering up on his dad's back and trying to wrestle his tennis racquet away, while Novak responded by picking his son up in the air as they carried on play fighting.Novak shares two children with his wife, Jelena – Stefan and four-year-old Tara. Just last week, he shared a post from a fan on Twitter comparing his forearm to that of his son.