Here's what one USTA coach wrote on the USTA web site:
"When training the players the USTA works with, we usually do some sort of "running" four to five times a week. The running session usually lasts between 20 – 40 minutes, but there is a lot of variety in the types of running we do.
You’ll note that we put running in quotation marks, because much of what we do is different from the long, slow distance running many tennis players are familiar with – there is some long distance running, but the “running” sessions also involve footwork/tennis agility work, or interval runs. The type of running depends upon the periodized strength and conditioning schedule of the player.
Generally, the long distance running and longer interval repeats (400s and 800s) are done during the preparation phase when you are getting ready for the season. Shorter, higher intensity intervals (20s, 40s, 60s, 100s, 200s, and 400s) and on-court footwork/tennis agility are the main focus during the pre-competition phase in the weeks leading up to main competition or competitions. During the competition phase of the season, on-court footwork/tennis agility is the “running” focus.
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."
High Intensity Interval Training (HIIT): http://www.intervaltraining.net/hiit.html
USTA agility drills: http://assets.usta.com/assets/1/USTA...oc_437_269.pdf
Stringing together several reps of an agility drill (like the spider drill) with swings at each change in direction in HIIT fashion can let you work on agility, recovery and conditioning.
Of course you can also approach your hitting sessions in a HIIT like fashion.