Every time I think about picking up a racquet again, I read stuff like this and have flashbacks and go work on my bike instead.
Hitting serves for an hour or doing 3x20 15/15s. 
I’m Sunday social tennis only (well maybe post covid).
Last straw was holding adult districts during the day in the middle of summer in 100 degree weather and 100% humidity. You either waited all day as your matches were canceled due to rain or nearly had a heat stroke while playing.