McKinnie -- who's listed at 6'8" and 335 lbs -- tells us his friends Venus and Serena have been putting him through some high-intensity tennis drills at a private facility in Florida to help his balance and footwork.
Bryant -- a Pro Bowl offensive lineman -- says the sisters focused on quick lateral movement ... as well as improving acceleration backwards and forwards from a dead stop.
McKinnie says the workouts were difficult ... but the Williams' were very patient with him and he feels it'll all pay off on the field.
And now that the lockout is finally over -- thank god -- we'll actually get a chance to see for ourselves.
