I have a very similar problem. I am in good overall health. I can play hours of doubles at a time, but when playing singles, I get winded real easily. One long rally and I feel dead tired. This has crept up on me in the past year since I turned 30. I never used to have such issues before. I do suffer from seasonal allergies, but I feel the same playing indoors in the winter here in Northern Virginia. Is this exercise induced asthma a real thing? I have never heard of it before. My doctor has never mentioned anything about this when I have gone to see him about allergies. I am 5-8, 170 lbs and have never smoked. Any advice is appreciated.
Don't be upset by the results you didn't get from the work you didn't do...