We have had 4 cases of Rabid foxes attacking in our county so be very careful. My boss had a rabid fox get through his fence and tried to attack him while riding his lawnmower. His lab (trained hunting dog) took off after it of course and ended up being bitten (thank goodness he was vaccinated) But he is a 100lb dog..I cant imagine what a fox would do to a dog the size of a rabbit.
Rabid foxes will not stalk though they will just attack so most likely he was not rabid...just hunting