I think my issue is game and point management. I try to chase every ball down.
I am fit but not young any more. Also, when I play against a very strong opponent who takes the time away from me, I should just let it go.
What I started practicing is taking my time a little bit better to recover between the points and between the games.
This is where I could still take some advice on the best ways to recover.
So if you think about strategy, you want to think about your ideal recovery spot based on the last shot you hit, relative to your opponents high percentage play.
Cross court rallies are your friend. You hit high percentage and you travel very short to recover. Just have a better cross court shot than your opponents

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Playing above will reduce your need to cover the other half of the court. If you take your backhand down the line from the baseline, you took your (assuming) weaker shot to your opponents stronger shot. And your opponents high percentage shot is cross court forcing you to quickly move from the ad to deuce side and increases the chances you go on defense.
Above is one way.
Another way is to just take more risks (smartly). Meaning… if it became a battle of attrition who would win? If your opponents fitness level is above yours, then you’d possibly be better off gambling more and thinking differently. As in… “how can I win this point in less than 5 shots?”.
It depends on your natural talents a bit, but it was an effective change for me back when I played tournaments. I hit well but was more out of shape (cardio) than most people I played against. At some point I realized that the likelihood of me winning a point or a match with long rallies is pretty low in that scenario. I have also been blessed with a bit serve and some natural power. So I just started going for more. Nothing to lose. I’d lose fast or win fast. But in a tournament scenario that’s better than losing slow or winning slow since match after match compounds.
You know… or you can do some extra cardio and turn that into a strength

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I’ve since changed from playing competitively to moreso for exercise. So for the first time in my life I’m playing more of counterpuncher style, which is interesting and new for me. But otherwise I don’t get enough cardio in.
Laaaaast thing I’ll add:
Equipment matters.
Lighter racket, looser strings, or more forgiving racket reduces the cardio fitness necessary to get into good positions and hit decent shots.
The heart rate difference between me rallying with my functional tennis saber (37sq in) vs with my 98 sq in regular frame is drastic. To get into position to hit a tiny head over and over and over is much more tiring than being able to get away with lazier footwork and poorer contact.
Even on the same racket. Tighter strings vs looser strings generally impacts my heart rate a bit. Looser strings…. Racket just does more work and is more forgiving. Can keep heart rate lower. Compared to tighter strings where I have to work more, especially when forced out of position a bit.