Rek'Sai vs Master Yi

Save Furious Bite (E) for when your Fury bar is full: the bite then deals true damage.

Come out of the ground with Un-burrow (W) under your target: they and nearby enemies take a knock-up, so you can engage or interrupt a channel.

Cast Void Rush (R) on an enemy you damaged in the last few seconds: untargetable, then unstoppable, you lunge at them, and your Un-burrow (W) comes back.

Cast Burrow (W) before crossing the map: your Tremor Sense reveals to your team nearby enemies who move unseen.

Dig with Tunnel (E) near lanes to gank and on your escape routes: each tunnel lasts and can be reused, but an enemy standing on an entrance destroys it.

Against Highlander (R), don't run alone: regroup with your team, because he gains speed and ignores slows.

If you have CC other than a slow, land it right after Alpha Strike (Q): during the strike, he's untargetable, and Highlander (R) ignores slows.

If you have CC, interrupt Meditate (W): he channels it to heal, more at low health, and to reduce the damage he takes.

Put pressure on his jungle early on: Master Yi is strongest once he has his items, and each stolen camp delays him.