When you have a dog you learn to distinguish their different behaviours, making it easy to tell the difference between, say, submissiveness and actual fear, or playfighting and aggression. Their body language is distinctive and easy to learn: noting their ear and tail position, hackles and teeth become second nature to you, so that when another dog barrels towards yours at top speed you don't even have to stop and think about whether it's a threat or not: you can tell, just as you can distinguish an angry face from a sad one.
So I thought at first that what I was watching in Dulwich Park today, only a few yards away, was two dogs playing with one another: chasing, but not at top speed, turning and running around together. One looked like a very large, lean Alsatian (we found out later it was a young Alsatian-greyhound cross); the other was much smaller, a sort of red-coloured, slightly mangy... fox!
These photos aren't very good, but I didn't have much time to work before the fox decided he'd had enough and went to kennel in a big patch of logs and brambles. The dog has his toy in his mouth (click to enlarge):
What neither photo captures is the clear impression that both animals were playing. Neither reached anything like top speed; both were at a loping sort of half-pace, and even when the dog caught up to the fox, as it did several times, the fox didn't turn and snap, or fight, but merely quickened its pace a little, or cornered, then looked round to see where the dog was.
I don't know what to conclude from the incident, but it was exactly the kind of behaviour I've seen from my dog when she's playing with another: letting it catch up, then leading the chase again.