Rui did not forget that despite all his strength, the Academy counted him as a grade seven Martial Artist, three levels below grade ten. Even Fiona was probably at grade eight or nine at most. She had mastered a lot of techniques, but her Martial Art lacked individuality. Her prowess came from her techniques, not the depth of her Martial Path.
One of the reasons he was able to cope as well as he did in their fight despite the large gap was because his mind had reached a higher level in the Apprentice Realm than hers did. His raw cognitive and neural enhancements were greater, it's just that she more than made up for that gap with a bunch of powerful and incredibly difficult techniques.