That's correct, they never show Riku defeat Roxas. There's something else too, the reason Riku was wearing a blindfold when he was fighting Roxas was explained towards the end of the game. He wore the blindfold when he used the power of darkness again so he couldn't see what was real. It was a way for him to block out what he knew to be true. It could very well be we saw Riku in that scene before he became Ansem again.

Riku rawks!!!