It was stated at the beginning. That's why many people were confused afterwards. Kishimoto made it seem that he was stronger than both Hashirama and Tobimara. The truth is they were going easy on Hiruzen on that battle, so that he can dispose of them. When Hashirama was declared to be the strongest Kage produced in Konoha. People were not really satisfied in seeing that to be the truth. Some people believe Kishimoto made a huge lie in the story or he was downplaying Hiruzen's character.