I had a similar situation when my dog met a black lab juvinile, not an adult, but not a small puppy.
The loose black lab just runs up to our male, while on a walk, the lab was a female, she was just playful but he went nasty on her, trying to put the dog in its place, the dog then ran home (my dog on a leash)
The next time we go by the lab (in an invisable fence), without hestitation breaks through the invisible fence, gets shocked and keeps coming after my dog, but in attack mode. My dog stand his ground and they mix it up a bit till the owner comes running out to restrain the lab.