Is Brock Lesnar "The Invincible Fighter"?

He's never lost a single fight except to Frank Mir which was obviously a fluke. Is it possible that this would be living legend may one day be defeated by another guy who is a better fighter? Comparing Bruce Lee the founder of Jeet Kune Do with Lesnar's natural wrestling abilities as well as size and raw strength, Brock would normally win the ground fighting or the slugging match. In order for Lee to beat him in a real life street self defense fight, he'd have to rely on his speed, and skill, keep the battle standing up and stay away from the big seemingly undeafeatable Lesnar. Brock is by far the more powerful fighter and will win a ground wrestling, or slugging match almost everytime. Bruce would basically have to take his time and wear him down and not trade with him. I respect Brock as he's a powerful fighter and a big guy too, but I also think he CAN be beat by anybody at any given time. So far, only Frank Mir has done so. Would it hurt Lesnar if he were to get punched in the groin in a real life street fight?