The longer they grapple, the more clear it may become that the Master's main aim seems to be less about a brutish, unfocused assault, and more about emptying the Doctor's pockets. He's sure the Doctor must have appropriated his screwdriver by now, somehow-- it's just the sort of irritating, presumptive move he'd make. As if everything the Master may find remotely useful is automatically his property.

He expects the Doctor to put up a fight, if a weak one, and fends him off fairly easily at first, rifling through his pockets in as efficient a manner as he can manage. But when the Doctor's pathetic flailing manages to connect with his apparent head wound, the Master freezes, sucking in a breath and seeing white for just a fraction of a moment.

It's probably just long enough for the Doctor to gain some sort of upper hand.