Very neat set! Even if a touch blurry, if you were wide open (aperture at its widest, f number at its lowest) and at the highest ISO then it must have been pretty dark so, yeah, pushing the camera to its limits (as wildlife often can).
As you say its play (and the cats ears are forward rather than back even when very close up to the fox) then chances are you'll get more opportunities. You might even try flash, in general most animals are pretty oblivious to flash. They tend to ignore it more than react to it - sound tends to have far more impact. However each animal is different and different experiences in their lives as well as their own disposition might well vary their reactions.