Original: Frieza's fears were realized when he was defeated by Goku, who in turn had become a Super Saiyan.
Simplified: 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.