Fae had grown immensely in power in the time that she had joined the Academy. Rui did not forget that even two years ago she was able to compete with Nel on roughly equal footing. She had grown vastly stronger than she was two years ago. That fact along with the ridiculous power she had demonstrated just then truly made Rui excited to fight her once more.
Part of the reason was that he had never actually beaten her in a fight yet. Thought that was also further because they hadn't had a serious proper spar in recent times. After Rui's third training stage, where he had mastered Outer Convergence, he had immediately begun under taking missions and when he returned, he had immediately jumped back into training for the Kandrian Martial Festival and the Martial Contest.
He simply didn't have an opportunity to spar against her.