Not having all that much room to move around in only adds to the fight, he feels. It makes it more interesting and in an effort to keep the good times going, Spike slides into gameface as he slides up on his feet. This is the only thing that matters, not whatever's going on in this freako world of corridors, not whatever got Jack spooked — this. Blood.
He does back up when Jack kicks at him, keeping just barely out of reach so the blows only grace by him if that. This retreat won't last forever but he figures it's only fair if they take turns.