Who would win in a fight? Rafael Nadal or Michael Phelps?

It's been an ongoing argument in my school for a while now. Some people think Rafa because he seems to have more muscle m and is known for his scrappy nature on the tennis court which he would without a doubt bring to a street fight. Some think Phelps because of his height and reach advantage. I think it needs to be put in the UFC octagon so a true champion can be discovered but I'd love feedback on this question. Thanks!