okay first of all, its master yi, if you don't know how to use him, you wont have the runes or all 30 mastery points.
the main point of this build is just to farm, destroy turrets, and simply win, with this build you should farm super easily, kill turrets with massive ease, and after 20 minutes of farming, kill champions without even trying!

I choose marks of desolation because they are the best for a melee DPS champion.

I choose seals of attack speed increase your chance of last hitting minions, some use dodge seals those too are good but in this build you need the money.

In my opinion glyphs of shielding will save your *** in more then one occasion, some people would choose glyphs of focus for cool down reduction, i find this useless because is you get a kill while using your ultimate it refreshes it automatically, and half of it for assists, so you have no need for cool down reduction.

Quintessences: i have 2 choices, flat armor pen, or flat health, either one is my choice, but i prefer armor pen because champions have less then 25 armor at level 1 so you're doing true damage with every hit. Combine this with wuju style and you can get easy first blood.

My masteries are standard melee DPS masteries, the reason i use cool down reduction, and yes this does sound like i am countering myself, is that a little bit of cool down reduction is a good thing, you can use your spells more, but you do not need to much, masteries are much more for early game and you do not crit very much early game so i use the cool down reduction.

I take improved ignite because it gives 10 ability power wish helps you farm with alpha strike.

I use improved ghost because ghost will save you in every game, multiple times, if you are backdoor-ing towers and you get ganked, ultimate up and ghost, no one can catch you.

Doran's blade is the best starting out item for DPS champions, 10 extra damage combine that with you armor pen and wuju style, it helps you get first blood.

I take mercury's treads, most would say Beserker's greaves, but with this build and using your ultimate, you have 2.5(max) attack speed so there is no need for extra attack speed, yes in the beginning it is much better, if you want to buy it and then later sell them for mercury's treads that works. Mercury's treads is very good because it combines with your magic resist glyphs, and if you gut stunned with out them, you are screwed. With this style of build that is the #1 way i die is to stuns, so i chose Mercury's treads, and that happens much much less.

Stark's fervor is recommended for master yi, it gives you(including the aura) 40% attack speed, 20% life steal, which reduces the from thorn mail to only doing 10% damage to you if your enemies have it. Stark's fervor also gives out the aura which gives your team 20% life steal and 20% attack speed, well that just helps everyone! and finally it reduces there armor even more, holy **** this thing's is perfect!

Phantom dancers, they are the basic attack speed item. they give good crti strike chance and extra move speed, and since yi is the fastest champion you can catch anyone.

The black cleaver reduces the armor (like Stark's fervor's armor reduction)of the champion you are attacking, does not cause armor pen. so this helps out your team get them killed, Plus it stacks with Stark's fervor for added killing power. some people would use last whisper but that only gives armor penetration and does not help your team, and does not give attack speed.

In this build frozen mallet is mostly for the health because you are the fastest champion it is easy to catch anyone. The extra slow just ensures that you get the kill if they run or have a speed boost like ghost!

Madred's blood razor, personally i have never played a game long enough to fully obtain this item because you destroy towers sooooooooooooooo fast the game ends relatively quickly, my fastest game with yi, no surrendering, i had 13 kills 0 deaths, level 18, everyone else level 14 ish, 18 minutes in, 9 buildings.

with all your items, runes, and masteries you are attaching through 95.93 armor and i believe that if you do get Madred's blood razor you could solo baron Nashor.

Late game a wave of minions will be gone in under 3 seconds with this build, and if you're lucky alpha strike will take them all out.

pros:
- fast, fastest base speed plus with ultimate you have bonus speed and cant be slowed.
- good early damage with wuju style
- easy to learn
- low mana costs so no worrying about
- good laner because of his healing capability
- good one on one fighter, some would say the best next to xin zhao, warwick, tryndamere, and jax

cons:
- no range so you have to be in battle to farm
- only 2 abilities do damage
- you might get focused.