Fragile Ace

Two guys live in an alternate world. They both play tennis, but one is a competitor and the other is a hobbyist. Nao (who plays tennis as a hobby and is 18 years old) used to be a big fan of Yuki (who plays tennis competitively and is 17 years old). Yuki has always been a prodigy and incredibly cute in every way. Since he started playing the sport, he has won almost everything he set his mind to. But no one knew that Yuki suffers from a chronic illness. His illness is such that every 3 months he has a period when he is completely weak (he can barely move) and sometimes dizzy. This lasts for about a week, and for some reason, after such a period, Yuki's appearance changes a little and becomes even cuter. Yuki doesn't like being seen as weak so he smiles and pretends to be fine. Suddenly, Nao and Yuki meet and Nao experiences what it's like when Yuki has this period. And they slowly fall in love.