I'd say my endurance is fine. When I'm fresh I can go for hours and remain pretty unphased. The problem becomes when I stack days like that on top of each other and I start to wear myself down. Even then, it's more that my legs start becoming less responsive than anything.
For example, take me at a lesson after a few days of playing long hours with a current futures player. There are points in that lesson when I get winded after drills that involve a lot of movement (crosscourt forehand, crosscourt backhand, down the line forehand, backhand volley, back to center, repeat), or I do a 100 ball drill. Following that, I took about 20 minutes to rest before playing a three setter with a slice heavy 3.5 player (involved a lot of movement) and around the third set I felt my legs beginning to give out (probably dehydrated a bit as well)
The short answer to your question is my muscles fail first and it's my legs that give out-- as conditioned as they are they definitely see a lot of work as I try to incorporate constant movement, split step and athletic stance through the course of training.
Ok, I went through the same thing. I strength trained, did intervals, ran distance, and it didn't help the problem. The strength training and intervals made me stronger and faster, the distance running helped my wind but no matter what I did my legs would give out. It seemed like if I got more leg power from training I used more in playing and I ran out just as fast. So after playing a few days in a row my legs were toast.
First thing I focused on was recovery. I got on a foam rolling and stretching routine, rolled myself with the stick and used tennis balls on the bottom of my feet, made sure I ate enough protein and drank enough water.
Then this winter I focused on training balance and muscle endurance.
Category 1: The Slow Burn.
For all of these things you shouldn't feel like you are taxing your heart or lungs so if your heart beats fast or your breathing speeds up, slow it down.
I did stairs, lots and lots of stairs just walking 20-30 minutes. We have a stair machine at my gym or do bleachers at a school.
Bike, turn up the resistance so you have to work to pedal at about speed 60. Do about half an hour, sometimes for the last 10 minutes drop the resistance down and bring the speed up around 90-100.
Step ups. Get the 2' plyo box and start with 25lbs in each hand do 12 with the left leg and 12 with the right, rest a minute and repeat until you want to die. You sound pretty strong so that might be too light for you. Basically you want the weight to where after the third set of 12 on each leg you are questioning your life choices.
Lunge walking and reverse split squats. Hold a barbell over your head for super fun times.
Category 2: Balance.
Single leg Romanian deadlifts. Great for your ankles, great for your balance, great for keeping you humble.
Bulgarian split squats, because suffering is fun.
Inverted bosu ball squats, I usually just use a 12lb medicine ball for balance, no heavy weight.
Single leg squats, just do bodyweight and get as low as you can.
Category 3: Jump around.
Box jumps, and try to land quietly.
Jump squats, kettlebells are great for these.
Get a little hurdle, maybe 6 inches, I use a yoga block so I don't kill myself when I knock it over, put your feet together and jump over it forward and back 50 times and side to side 50 times. Rest and repeat.
Get a jump rope and an agility ladder, go nuts for half an hour. With the agility ladder go slow until you learn the pattern. Better than rushing and messing up.
Switch it up and mix it around to keep from getting bored and to keep your body guessing. I would do about an hour per day, every other day is ideal but twice a week is fine.
J