When dogs are displaying playful fighting and romping around behavior, it is truly wonderful to watch and a great benefit mentally for the dogs. We call this rough and rowdy play and it is actually essential for dogs to have this type of play in order to learn play signals and play boundaries. As puppies, dogs teach each other when biting has become too hard (bite inhibition) by correcting the other puppy when it bites too hard (they do this by immediately walking away from the offending puppy, who then gets the message very quickly that if it wants playmates, it can’t bite that hard!). This is much the same method they use when teaching each other play boundaries (although they use many different signals) and is why it is so important for all dogs to be able to play with abandon but always with supervision!
In much the same way as with children playing, where laughter can quickly lead to crying or fighting, dog play can quickly turn aggressive and you need to know the signs in order to diffuse it before someone gets hurt. When growling starts to sound firm, guttural and threatening, whenever teeth are shown and there is a fixed stare, whenever a dog becomes overly-assertive over another dog, pushy or bullying or pinning it down, this has now gone from playing and become aggressive behavior.
It is now time to quickly separate the aggressive dog from the submissive one, as this type of aggression can explode very quickly to an attack. I would immediately distract the aggressive dog from the other dog by calling it to come to me, or clapping/whistling loudly for it to come, then redirect the aggressive behavior to another more desirable behavior such has sitting next to me quietly to decompress for a while. Reward the good behavior and make sure the dog does not start up again. If it persists, remove it from the area for a moment then reintroduce. If it still persists I would take it away from the immediate area and the other dog completely.