Even a UFC fighter would stand no chance against a large chimpanzee. These animals have strength and it's not like you can time them for a clean strike. They won't follow any fighting pattern that any man has ever trained against.
They move fast, unpredictably, can grab you with all 4 limbs and on top of that have massive canines. They also will not follow any sort of fighting rules, they'll claw, bite, punch, kick, pummel, jump, bend/break limbs with a single snap, gouge eyes, claw, etc. Truly terrifying.
Just the savage nature of a wild animal vs a human is something most cannot comprehend.