Master Yi vs Yasou

Well seeing as Yi is the king of 1v1ing people not named Jax...

Master Yi would win because Yasuo keeps saying what he is going to use next out loud.

it all depends on items, skill level, timing and such. if gets his ult while has his ult up, that puts yi at a huge disadvantage due to the AS buff that he has during that, wasting almost half of that timer. however if yasuo misses that, yi will probably win due to his E true dmg, double strike, and his W if he messes up

Technically, they are tied with some others as the greatest. You forgot Fiora, Talon, and Katarina.

Yas is not really a dualist/ melee. He uses his E to create space and then steel tempest to strike (both are critical). Between good yas vs Yi, it's tough to say. But yas has an upper for he can strike from distance.

If there are no minions around for yas to use then Yi WILL win.