I find I am simply _unable_ to apply heavy topspin against someone who hits hard.
If he hits heavy topspin then I have to take it well on the rise, which means my ball will be rolling backwards as often as forwards.
If he hits it hard and flat, then I cannot time time the shot if my racket is moving perpendicular to his balls flight. So light topspin is all I can manage.
If my opponent hits soft to medium pace shots (so I can take them after they start to drop without retreating way behind the baseline), then I can load up on the topspin.