LONDON, Sept 20 (Reuters) - Ukrainian Oleksandr Usyk says he is more worried about his son"s first days at school than stepping into the ring against world heavyweight champion Anthony Joshua in London on Saturday. Joshua will be putting his WBA, IBF, WBO and IBO belts on the line in the fight at Tottenham Hotspur stadium. Both men won gold medals at the 2012 London Olympics, Joshua the champion in the super-heavyweight category and Usyk at heavyweight. "At the moment I don"t think about Anthony Joshua," The Sun newspaper quoted Usyk as saying. "I don"t think about what he will do, what he will use or how he will box. "Right now I think more about how my son feels at school because he has just started first grade and he really did not want to go and do it." Usyk, 34, is unbeaten professionally and has stepped up from being the undisputed cruiserweight world champion. The WBO"s mandatory challenger flew into London on Monday. Joshua, 31, told fight broadcaster Sky Sports television that he did not consider Usyk to be someone boxing above his preferred weight class. "All boxers can move up in weight. Some feel better at the higher weight, they carry more power and feel fresher," said the Briton, who has a professional record of 24 wins and one defeat. "In my mind, I have removed the conversation that he is a cruiserweight." Joshua is fighting Usyk after the Briton"s planned title unification with WBC champion Tyson Fury in Saudi Arabia was called off due to an American arbitrator ruling that the latter"s rematch with Deontay Wilder took precedence. Fury is due to face Wilder in Las Vegas on Oct. 9.