pushing you into anaerobic zone often or totally not by mainly developing one hand only and in stop&go fashion ?
I play doubles on my off day. 
FWIW, I played college tennis.
Unless you are 4.5 or above, or play singles against a consistent pusher, then you aren’t going to get much compared to a good 5 mile run. I only run about 3 miles per match when I play singles, and only half that amount is worth counting. Also, I can play tennis at 175 lbs, but race triathlons at 155-160 lbs, so, obviously the training is very different. I know Roddick said he did a weekly 10 mile, hard and hilly, training run to stay fit for those 5 set matches. But, at his level, the game is entirely different…
If you want to get a great workout that will feed into your triathlon energy systems, then get a ball machine and have it set for both corners. If you get a machine that will hold 300 balls, you will get a very decent cardio burn.
But, specificity, specificity, specificity. Does Carfrae play tennis? 
I’m guessing not…
-Robert
pushing you into anaerobic zone often or totally not by mainly developing one hand only and in stop&go fashion ?
Have you been reading Captain Canada’s training plan?
I’ll second what Robert said. Doubles is worse on your joints, but singles can be a decent anaerobic interval workout if you’re both hitting well.