Freeza and Goku are matched up. Freeza sends Goku into an area that's made to be similar to his planet. In it, those who aren't apart of his race suffer side effects like oxygen issues and climate change. And for some reason, you age faster. So Goku was aging as the fight progressed, and getting weaker while Freeza had the full advantage. Goku finished him off with a One-Handed Kamehameha tho.