LONDON, Sept 23 (Reuters) - Ukrainian challenger Oleksandr Usyk on Thursday dismissed concerns that he did not possess the size and strength to take out world heavyweight champion Anthony Joshua, saying that the "power is in your spirit". The Briton will put his IBF, WBA and WBO belts on the line on Saturday against the undefeated Usyk, a former cruiserweight who is 18-0 in his professional career. "People talk a lot... we"ll see on Saturday what happens... because the power is in your spirit, it"s not about being big or small," said Usyk, who has fought only twice before as a heavyweight. While new to the heavyweight division, Usyk tore through the cruiserweight class after turning pro eight years ago, becoming one of boxing"s rare unified champions in 2018 by winning all four belts in the category. "... David Haye was three times smaller than Nikolai Valuev which shows that it doesn"t matter," he said. The 34-year-old added that he expected a chess match in the initial rounds of the bout. Joshua, who last fought in December 2020 when he knocked out Bulgarian challenger Kubrat Pulev, said: "Happy to be back again, you have to want to train and put the work in and improve and practise... I"m not an easy fight for anyone, I like fighting. God has blessed me, shown me the path to get into boxing." The 31-year-old was slated to take on compatriot Tyson Fury in Saudi Arabia this year but the fight was cancelled after an American arbitrator ordered Fury to take on Deontay Wilder in a third bout. "This fight gives me the motivation to practise. I want to give more and more each day. I"m still in camp now, this is just another day in camp... if you tell me I was fighting King Kong I would give it a go. This is my job," said Joshua ahead of the bout at London"s Tottenham Hotspur Stadium. Usyk said that the fight had the potential to make history. "I"m grateful that this is happening... people will remember this fight for a long time." Joshua added that he had studied a lot of his opponent and was a fan of the Ukrainian"s style of boxing. "(Usyk) was fighting 10 or 12 years as an amateur before he went to the Olympics, and the worlds, so he is probably happy to be in this position -- the cream always rises to the top."