When i first saw Yi, i looked at his ult and his third and though that he was a physical dps period. When i saw his first ability i i thought it was perfect for chasing in combination with his ultimate. And his second ability... i though it was rather useless. So that was it. But later on, with more experience as a player i saw how incredible his ap scaling was. 100% of ap added to your Q and 160% to your W. I was surprised to see that but only one thing was for certain: Master Yi is an incredible dps mage. Later i found out that at early game, AP Yi sucks, but AD Yi PWNS!! So i came up with a way of dealing with this problem, which i will explain later on.

Since I am going for AP, i will go for magic penetration marks, flat mana regeneration seals(yi has a terrible mana pool and mana regeneration), ability power per level glyphs (we want that ability power to scale a lot) and magic penetration quints(to get maximum magic penetration).

Since i am going AP, the only things that I really need is the cooldown reduction, the little extra ability power(added to my runes) and the magic penetration. After that, the rest of the offensive tree becomes useless. Then I choose to go for 21 points at the utility tree because Yi has a very big mana problem and he will need all the mana regeneration and mana pool he can get. As well as some nice movement speed to improve your chasing ability even more.

My best choice usually is Flash and Exhaust. Sometimes Yi gets beaten preety hard, and he needs a quick way of scaping before its too late or maybe a way to grab a kill in tha last second and needs to be a little bit closer to land his Q. Here flash is just perfect.

I pick exhaust because of people like Teemo, Sivir, Singed, Mundo, who increase their movement speed, I also use it for people like Twitch, Teemo, Ashe, Miss Fortune, who do a lot of damage with their physical attacks and focus Yi quickly because they notice that your are just dominating the game. With exhaust you make them preety much useless and gives you the time to finish them off before they even touch you.

Note: If you dont like one of them and you think that the mana regeneration runes could be replaced with another seals that better fits yi, you can always go for Clarity as a summoner spell.

Now, i put these three together because they complement eachother nicely.

Early game

Right now, you have two choices:

1) Go AP and suck really hard

2) Go AD DPS and kick major butt.

So right now you will forget about ap, and you'll focus your items on attack speed at first(stinger). This will leave you with no attack damage but guess what!! You got a passive that gives you an incredible damage bonus since level 1. Letting you start with the higher physical dmg in the game. Thats why you will priorize your E on top of your other abilities. At first you need to be quick, go last hit, get out. Go, Q, last hit(if necessary), get out. And so on, thats why you go for boots of speed an the start. And if you are bad at last hitting, Yi's bonus damage can be perfect to help you with that. And if you want to harass you can always use your Q to catch up, and then use your E's active, to do a little extra damage. Also take in account you double strike and try to save it for hitting champions. Because of this you are gonna need potions so its good to take a couple of them the first 3 or even 4 times you go back to base.

When you hit level 6, enter killing mode and go Q your objective, inmediately after hit E and R yourself and get kills, kills and more kills. If you are low on health in the end and you feel in danger, go to a brush and use your W. Then get the hell out or get more kills if possible. Remember you are squishy so dont get greedy. This is a crucial part in which you need to get yourself as fed as possible, farm minions, get kills and assists.

If you are doing good, and your lane is doing great, it would be a great idea to gank middle lane when the enemy champion is close to your allied turret. Always remember to ping the map so your ally knows that youre planning a gank. If you dont tell him, he may be way far from you when you start ganking and he may no notice. If your middle laning ally has a slow/stun/fear or any crowd control effect, it will be good to ask him to initiate the fight with it so you have more time for dealing damage.

Hopefully, you just got a stinger and you completed your sorcerers boots so your ultimate has a better chasing effect and you also got more magic pen for your Q which will be at rank 1 at this point.

From now on, you go hybrid. YES HYBRID! Get your sheen at quick as possible, it will increase you Q damage and your W heal incredibly. Also, because of its passive it will make you hit hard after your Q. I sugest activating your E and then use your Q so that when you hit with your normal attack, sheen's passive is applied with your E bonus included. A good idea would be priorizing your Q and W qhen you level up.

Mid Game

Here you will now focus entirely of AP Yi, HERE COMES THE REAL PAIN!!

Go straight for Rabadons Deathcap. With that your AP will scale incredibly high, so now its more convenient to have a lich bane than a sheen. So add that extra AP, mana and movement speed(convinient for your ulti) from lich bane and its passive. Now your damage output will be tremendous. Because of rabadons and your lich bane, your Q will deal a lot of burst damage, and with lich banes passive, your next hit will leave your oponents with no HP. Note that you should try hit your Q to 4 champions at once so that you get full benefit from your burst damage and then take out the enemy's squishy dps with you physical attacks.

So now, you will notice you need to actually spam your Q a bit more at teamfights, so a great thing to do is build your stinger into a nashors tooth. It will give you 15% more Cooldown reduction(the other 10% is from the stinger) more AP (yay more dmg and heal) and finally, 10% more attack speed. How's that?

Late Game

Right now you should be just dominating the other team. But there are a couple of issues still to be solved:

1) Survivability: You will get a rod of ages, which will give you enough HP to actually keep up the fight. If you want, you could replace this item with a Frozen Mallet, it will give you better chasing ability, though I think with exhaust, flash, your ulti, and your Q you have a lot already. But that mallet will gave 20 of dmg and a good amount of health, depends on your playing style really.

2) Mana issues: You'll notice that you kills give you the oportunity to use your ultimate multiple times and even spam the rest of your abilities. But thats useless if you dont have mana, that's why you get lich bane in the first place, but... still no enough. So then you get a rod of ages. Now you are okay, but if you really want endless amounts of mana, go for the archangels staff after the rod of ages.

3) MORE AP: You'll get a lot with rod of ages, but the real deal here, is the archangels staff. It not only uses your mana to generate more ap, but it also generates more than 1000 mana and uses the mana from the rod of ages and tha lich bane. Like it? Theres more, imagine that amount affected by rabadons deathcap passive. Now... you do more damage than Ryze himself xD

IMPORTANT: Always remember that your W is not only for when your are being focused by the enemy team. An alternative is using flash to escape from the team fight with 100 HP(for example) and then come back into the fight 5 seconds later with full HP.

I hope this guide was usefull to anybody out there, I am peruvian so maybe my english is not quite as perfect. Sorry if there were any horrible grammatical mistakes. I would appreciate any comments suggestions and feedback for this guide. And sorry for the inmense walls of text u.u