It depends on how good shape you are in, how "beaten up" your body feels and how much you are willing to push yourself.
The "best" from a pure tennis point of view would be to do mainly HIIT work if you are already in great shape.
Some have to "work into" doing longer runs to build up a base at first before being doing more HIIT.
I left out the next line from that USTA coach above - but it applies to all of us.
"Recognizing that each player is an individual, we adjust the plan depending upon the player’s cardiovascular endurance, agility and their physical and physiological strengths and weaknesses."
Note that the HIIT principle can even be taken to doing agility drills to move better on the court (and even in our approach at hitting practice).
USTA agility drills: http://assets.usta.com/assets/1/USTA...oc_437_269.pdf
It is a general principle that the closer the exercise mimics our sport, the more we will directly benefit.
So, for example, doing spider drills and vertical and horizontal repeaters at high speed we are training out bodies to start and stop and change directions - just like during a tennis match.
Plus these have the additional benefit of strengthening the muscles that stabilize the ankles knees and hips so are better prepared to avoid injuries on the court that running in a straight line won't prepare us for.
That also reminds me that at some gyms pick up basketball is going on - a great sport cardiovascular conditioning, plus great training in those starts, stops and change in direction - particularly if playing defense and zigging when the other player zigs and zagging when they zag.