Simplify the following text for a Grade 7 native English-speaking person.
Original: Goku and Frieza then spar , with Frieza realizing that his opponent is the son of the Saiyan that gave him such trouble years before . Goku and Frieza are an even match for awhile , until Frieza reveals that he had only been fighting at a mere fraction of his full power .
Simplified: Frieza 's fears were realized when he was defeated by Goku , who in turn had become a Super Saiyan .