
Reviewed for patch 7.3d

Favorable matchup
Reviewed for patch 7.3d
Master Yi wins fights with basic attacks: Wuju Style adds true damage to them and Double Strike hits twice. Rammus punishes exactly that. Curled up, he returns damage to whoever attacks him, and Frenzying Taunt makes Yi keep hitting him.
Defensive Ball Curl greatly increases Rammus' armor and magic resist and returns damage to enemies who basic attack him. Yi needs to chain attacks to bring Alpha Strike back sooner and to trigger Double Strike; against the shell, every one of those hits costs him health.
Highlander makes Yi immune to slows, so Powerball and the aftershocks of Soaring Slam do little to stop him while it lasts. Frenzying Taunt is different: it forces him to attack you, and if it catches you curled up, he takes damage back on every attack for the whole taunt.
Wuju Style adds true damage that armor doesn't reduce, so the shell isn't a wall. What decides it is the damage return and the timing: while Yi is taunted he can't choose who to hit, and your allies get those seconds to punish him.
Levels 1 to 4: use Powerball's speed to pressure lanes while Yi farms. At dragon and in fights, taunt when he comes out of Alpha Strike; while he's untargetable you can't select him.
On Master Yi, never attack a curled-up Rammus: switch targets or wait for Defensive Ball Curl to end. Save Alpha Strike for when he goes to taunt you, because while you're untargetable he can't pick you. Farm on the side away from his pressure and join fights once he has spent the taunt.

Favorable matchup
Reviewed for patch 7.3d
Master Yi has no crowd control: to defend a lane or contest an early objective, all he has is his damage. Jarvan IV plays in that window, with Demacian Standard and Dragon Strike ganks that knock up, and Cataclysm to shut him in when he reaches the fight.
Demacian Standard plants a flag, and Dragon Strike cast toward it pulls Jarvan along and knocks up enemies in his path. It's crowd control that doesn't rely on slows, and Highlander only protects Yi from slows.
Our analysis: Yi gets stronger the more he farms, and his kit gives him nothing to punish a gank with. Jarvan IV can spend the first levels pressuring lanes and contesting the first scuttle and the first dragon, while Yi chooses between farming and answering without crowd control.
Cataclysm leaps onto a champion and raises an arena of impassable terrain around them, and enemies inside deal less damage. Drop it on Yi or on the ally he jumps on, once he has used Alpha Strike, so the fight happens inside your walls.
Levels 2 to 4 and first dragon: the flag combo is worth the most while Yi has no items yet. Later on, don't look for solo duels and keep Cataclysm for the team fight.
From Master Yi's side, the plan is not to give away the early levels: farm on the opposite side of the map from his ganks and warn your lanes that you have no crowd control to defend them. When you see the flag, wait for Dragon Strike before you go in, and save Alpha Strike for his Cataclysm.

Execution matters
Reviewed for patch 7.3d
Against Master Yi, Amumu has the tools to stop him. Bandage Toss stuns him and Curse of the Sad Mummy leaves nearby enemies unable to attack or move. It hinges on Yi jumping in with your team close by: alone, Amumu can't finish him.
Curse of the Sad Mummy wraps nearby enemies and stops them from attacking and moving. For a champion who does everything with basic attacks, that's time with no damage and no Alpha Strike cooldown reduction.
Tantrum permanently reduces the physical damage you take, and its cooldown drops every time you're hit. Yi attacks nonstop; if he stays on you, Tantrum keeps coming back.
Bandage Toss stuns the target and pulls you to them. If the ultimate misses him, the bandage gives you another crowd control to stop him when he chases your marksman.
Look for the fight at an objective or in the jungle with your team close, and wait for Yi to come out of Alpha Strike. In isolated duels in your jungle, step away and farm the other side.
On the other side, as Master Yi, track Amumu's ultimate before going in. If he has it, let him spend it on your allies or come in from an angle where he can't catch several of you at once. Alpha Strike makes you untargetable: use it to try to dodge the bandage.