I love this. Why would a really big evil that attempts to destroy the world be aware of a single puny human trying to stop it?
Does the comet approaching Earth know of the astronaut that tries to deflect it? No. Does that make the story less gripping? Certainly not!
While most tyrants will be aware of the resistance in general, they will not necessarily be intimately familiar with every individual opposing them. Think of your story from the tyrant's perspective:
Many individuals oppose them. Through his police and secret service the tyrant watches them. He tries to control them, by putting those into jail that he thinks most dangerous to them. But eventually one of those that he was not aware of or could not apprehend will kill him. That is the hero.
From the tyrant's perspective, the hero is just one of many. Before he succeeds, he is not the hero. He is like one in a million sperm cells. One of a swarm of gnats. One of thousands of infections in the air you breathe every second of your life. And when he succeeds – when one sperm cell fertilizes the egg, when one gnat stings you, when you catch a cold –, it is not because it was this individual person's (sperm cell's, gnat's, virus') destiny, but because one of them eventually had to succeed, because if there are enough gnats, you will get stung. Is that the gnat hero? No. It is pure chance.
Only from the perspective of the hero, does he appear as the hero. Think of the tale of one gnat and what obstacles it has to overcome to eventually drink blood – and get away alive! –, while all around it its peers are being eaten by frogs, poisoned by insecticide, and squashed by huge human hands. Of course you are aware of the gnats trying to suck your blood, but are you aware of the gnat hero that succeeds? Not in the Russian tundra, with tousands of them circling you. Until it stings. Then you know. And from that moment onwards, all the tyrant's power will be fighting the hero.