Serena Williams' baby will be one the most physically gifted super-athletes of ALL TIME ... so says Glen "Big Baby" Davis.

Here's the deal -- Serena is arguably the greatest female athlete who ever lived. Her fiance, Alexis Ohanian, is 6'5" and smart (he's one of the co-founders of Reddit).

And with their super-DNA combined, Davis says he's convinced the result will be an ultra-smart, ultra-talented, insanely gifted Bo Jackson-style baby.

No pressure, kid!