I have been playing AP Yi for a while, and really he does decent damage with his abilities, but his attacks are not as strong as i need them to be, and the cooldown for my abilities are a bit to long to be spamed. I mean, Yi has the cooldown reset with his ultimate but for that you need to get kills instantly, and his only way to make his ultimate work is with attack damage. Also, a big flaw on AP Yi is the lack of attack speed, even with nashors tooth and his ultimate, it is not enough. But still, his second ability is really valuable and the ap scales beatifully in Yi. As well as for the damage that his first ability does. This is why i decided to make a Hybrid Build for Yi, with a higher focus on attack speed, cooldown reduction, more damage and armor penetration but also the necesarry AP to make his first two abilities work as they should. As a plus, I thout of a way to give him a huge amount of life steal and spell vamp.

I chose armor penetration marks instead of magic penetration, because Yi's main source of damage is his basic attacks, and your ap is used more defensively than for offensive purposes.

For seals, I like to go for flat mana regeneration because this guy has huge mana problems early game and his abilities consume too much mana. Later on this problem is solved but for the beginning of the game, you really want that mana regeneration.

For Glyphs i use cooldow reduction. This is an important aspect on Yi's build, he needs to use that 3rd ability more often for more damage, and also his second ability es very usefull in long teamfights when you are taking a beating.

For Quints, I also go for armor penetration. I like leaving my enemies with a negative armor value to maximize my damage so this combined with my build is very convenient.

Here I go for standard DPS masteries in the offensive tree, but then go to utility for some good mana regeneration and mana pool increase.

I like going for exhaust to slow my enemies down for a while as I speed myself up with my ultimate. I also use it when another dps tries to attack me so he has no way of escaping and instead of getting me killed, y get to reverse it.

As I already have an speed up and antislow, I can still get stuned, feared or taunted, so for those cases I grab my flash. I sometimes use it offensively for catching up when my ultimate and my Q and for situations when my precense there is extremely required.

Early Game

As you already have an amazing attack bonus from your E passive and your double strike, you should now concentrate on getting more attack speed. You'll get that from the stinger and the Beserker Boots. It will also give you more mobility in early game. In this phase you really want to get as many kills as you can, because you are kinda op at the begining with such a significant damage output from your E and from your attack speed. Inmediatelly after your boots, finish your nashors tooth to give you some ap, mana regen, and valuable cooldown reduction.

Try to get many last hits (minion kills or cs) with your alpha strike and if you want to damage a champion, alpha strike a minion close to your enemy, and you will end up in a much safer position. Try not to get too much damage. Just the right amount so you can use your potions and heal yourself. When you have the mana regen from nashors, or the fiendish codex, start using more your abilities, especially for survibility. A good way of doing damage is waiting for your double strike to charge up, activate your E, then alpha strike your enemy and attack him with double strike and then get out or if you can get the kill activating your ultimate right after you landed your alpha strike.

Mid Game

At this point you will have 70% attack speed bonus. So now, lets maximize that and get some damage with the Bilgewaters Cutlass. Always remember to get it first that hextech revolver. You need damage first and some lifesteal. After you finish the Gunblade. The rest is preety situational.

If the other team's tank is stacking a lot of health (cho gath, mundo, Sion), get madreds bloodrazor and use their health against them. Even if they got a lot of armor and 5k HP, your madreds will give you magic damage, so all that armor will be useless. And conveniently, the spell vamp from the hextech gunblade will work as a life steal for your madreds damage. After this, it is up to you, if you feel you want more damage, get an infinity edge. It has been buffed amazingly and Yi can take a lot of advantage from it. If you want to get through the armor that is getting in the way of your own damage physical damage, then go for The Black Cleaver

If they went for more armor and their health is not enough to really take advantage from it with the madred's bloodrazor, get starks fervor for the armor penetration and life steal life steal. If they are now dieying quickly go for infinity edge. But is the armor is still an inconvenient then get the black cleaver and problem solved, though I think this will no longer be necessary.

This is the part when most team fights happen. Your role is to wait for your tank to initiate the fight and then attack the squishy DPS and mages in the other team. If you get focused too hard get out of there, if you cant use your maditation, if you are still in trouble flash out of there. Walk around until you have a better positioning, and kill the rest. Dont get in the middle of everyone, always stay close to your tank, and if you think you can handle the situation, go ahead and get separated. But keep in mind that you are very squishy. If you can get out of the fight. Use meditation, and go back to the fiht with full health. Thats Yi's role, get it and out of the fight. Deal some heavy damage when he is inside and regenerate when he is out to get back there and do more damage. If you manage to kill someone, you can use your ultimate and your E again, and get more kills. Yi is amazing for pentakills. Remember, always leave the tank for the end, unless you got the tank in a really bad positioning and your whole team is going to kill him without any disadvantages. Attacking a tank is useless if the other team scores an ace before you even get rid of the tank. It is better to get rid of the main source of damage to then attack their tank. Sometime the best offensive is the best defence.

Late Game

In both cases get another Hextech Revolver as a 6th item, if you got the starks fervor you'll have 70% life steal if you combine it with the two hextech Gunblades. If you got the madred's bloodrazor, then you got 50% life steal AND spell vamp.

Your role here doesn't change much. You have more life steal so you can stay inside fights for a longer time or kick butt 1v1 and end up with full HP.

I hope this guide is usefull to anybody and if you want to give any feedback or comments to make my guide better be my guest :D I am peruvian so please forgive me if I had any grammatical mistakes btw xD

Please dont forget to rate :D