Serve and volley tennis takes a lot of energy too, no doubt about it. As some posters have touched on, including Urban above, serve and volley tennis can be very demanding as well. It takes a lot of explosive speed and strength too. You end up covering a lot of distance and it's that fast back and forth type of running (sprinting), with plenty of lateral movement mixed in as well. Don't forget the heavier frames too and what that does to your arm over the course of a match. Players like Rosewall, Laver, Roche, Emerson, Borg, Vilas, etc. (the list goes on..) were all former players that worked their tails off in training out there on the courts for many, many years, starting at a very young age. They were all extremely fit and just plain tough customers. Go out this weekend and run like this for about 30 mins and see how it feels.
Laver vs. Roche in the 1969 Australian Open SF