Aerobic training trains the system which fuels activities such as walking and jogging i.e. activities that are submaximal and those that are not really part of your normal mode of play.
However a sound base of this fitness is needed as it helps you maintain high levels during matches by aiding in the recovery of your anaerobic energy stores in between points.
Think about it, there is a lot of walking in between points as well as all the heavy breathing after a long point.
This is a great aerobic drill you can do practically anywhere!
I shot this video on the stairs that lead to the gym - another example of not needing to go into the gym to get some great tennis training and fitness done!
The problem arises when people try to improve their conditioning by getting involved in long distance straight line running which is not specific to tennis.
It can even have a negative effect on your
speed
and
strength
.