If you go on the court wearing clothes that don’t fit you correctly or are not suited for the weather, that can become a distraction that takes away from your focus on hitting the ball. For me, I also prefer the material that is designed to “wick away” the sweat from your body (e.g. Dri-Fit), to help in the cooling process.
At the end of a long, hot match, wearing a fully-soaked cotton shirt sticks to your body and feels like it weighs ten pounds. So you should carry extra shirts in your tennis bag to be able to change shirts when you are feeling weighed down.
In cooler weather, I would wear a wick-away shirt under a long-sleeve (usually also wick-away material) to keep the upper body warm. I also like to wear a sweater vest for those in between temperatures.
Again putting fashion aside, colors matter. Wearing black or any darker colors will really absorb the heat of the sun; so you want to avoid them on hot, sunny days … and use them on the cooler, sunny days. Conversely, white tennis clothes will help reflect the sun’s heat.
I am also a believer in “confidence clothes.” Remember when you were in business and had a “favorite tie” or the special suit that gave you an extra spring in your step? Well, tennis clothes can help give you that same feeling when you walk onto the court for a big match. Every little edge can help!