I'm a Master Yi main. I started playing season 3 but only really became invested in it at the beginning of season 6. I don't claim to be the best player, but I love Yi and I do spend a lot of time working over builds creatively to find new ways to play champions and take them to their highest potential.

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Since they nerfed Highlander and buffed Ghost, Ghost is preferable to Flash. Use it to close in on your target instead and only activate Highlander if you're certain that you'll get a kill or assist in the next 7 seconds.

AS helps with your clear speed immensely and Yi's kit scales off of it well.

5% CDR at level 1 and 10% at lvl 18. Combine with Black Cleaver and Spirit Visage for a clean 40%.

Health seals because you need some tank. Scaling because you have enough health to survive the early game already (it's mostly PvE anyway).

Fury: AS is Yi's most important stat.

Double Edged Sword: 1v1's are Yi's specialty.

Vampirism: Lifesteal helps early jg clears and late team-fights.

Bounty Hunter: Yi's an assassin with no CC.

Battering Blows: Deals primarily physical damage.

Fervor of Battle: Only other serious options for keystones are Warlord's Bloodlust and Stormraider's Surge. Neither of these give Yi any more damage and they require him to deal lots of it. In MTG we call this the "win-more" strat: useless when you're behind and superfluous when you're ahead. Thunderlord's Decree is interesting, but is meant for burst-poking and doesn't scale as well for Yi.

This took me a while to figure out, but now I'm confident I know what works best. Start with machete and 1 pot at Krugs no matter what side of the map you start on. (Fun Note: You don't need a leash once you get Yi's clear mechanics down.) Clear only small camps, no buffs. Ward each buff as you leave it if you suspect the early counter-jungle. Use your pot after you hit level 3 and meditate. This will allow meditate to heal for the max amount. Back after the last small camp and you'll have exactly enough to buy Skirmisher's Sabre. Then do a full clear (including buffs) starting Krugs, camps will respawn a little before you get to them. Now you should be at >80% health assuming everything went well and holding at least one buff. The rift is your oyster, take advantage of easy ganks, help protect towers when your laners die, and consider counter-jungling if you know where the enemy jungler is.

Tips for healthier clears:
1) Wait for monster to begin its attack animation before using Q (Alpha) to dodge it. This helps you get better at dodging skill shots and forces monster to waste time.
2) Only use E (Wuju) right before you double-strike and after the camp is at half health to get the most out of it and preferably don't Q while it's active.
3) Use W (Meditate) liberally when you have blue buff and otherwise save it for when you're below 50% health so it will heal more.
4) Check if monsters run somewhere when you're in alpha strike. If they do, they're running in the direction of another nearby champion.
5) As tempting as it is, try not to let anyone engage on you during your first clear. If they do, disengage with Ghost if you have to and ping for assistance.
6) Dodge dragon attacks with Q only when the projectile is in the air so that you dodge the damage and don't soft reset.

Watch one of Cowsep's games to pick up what I'm talking about here and more. He's even more of a Yi die-hard than I am.