By that point Skywalker butchered the Count, however, and he regained his stamina as per the novel upon Anakin's anger-boost.
Prior to said anger-boost, Skywalker was still driving him back convincingly. Granted, prior to that, there were disadvantages on the Count's part, but by the time the two went up the stairs they were mostly taken care of and yet still Skywalker's superiority was made clear.