Jimmy Connors in high heat/humidity would soak (repeatedly as needed) a bandana in ice water and tie/wear it around his neck.
This probably belongs in "Health & Fitness" but since "hot days" is the issue at hand, here's an old football trick (esp. good for you Bruce Willis/Michael Jordan baldsters out there--it does work): players sometimes "froze" cabbage leaves and wore them 'underneath their helmets. You can do the same wearing a well-ventilated, micro-fiber hat. "Freeze" some cabbage leaves in a freezer, then bring them with you on court by storing them in a mini cooler containing an ice pack. (I'm serious about the following): "Wear" the cabbage leaf underneath a loosely-fitted cool mesh hat. The hat protects you from the sun and the chilled
cabbage leaf will more than compensate for "stored heat" generated from the use of wearing a hat. The turgid cabbage leaves are "sturdy" to the point that they won't wilt as readily as say, lettuce; and in "staying strong" they'll "hold" the chill much longer. The loose-fitted hat allows for a pocket of "chilled" air to float near your nogin. A tight-fitted hat pressed flush against your scalp will "squeeze" the chill out of the cabbage leaf, i.e. you're trying to chill the air
as well. I admit it sounds strange but what if you have no choice as to when you're going to be on court on a hot/humid day (e.g. scheduled to play a tournament match)?? So what if you look like a cabbage patch knucklehead, it's still an effective way to help you chill out
. And what's more, you can always use the (non-used) "leftovers" for coleslaw.
If you feel like you may start cramping, here's another bizzare football-in-the-heat remedy (that is, if your stomach can hold it down): drink a small glass (6oz.) of chilled "non-sweet" pickle brine. The brine not only contains sodium but also contains chloride, a much needed element that goes to pot (along with potassium, zinc, etc.) when your electrolytes start to fizzle. Don't drink "bread 'n butter" (sweet) type pickle brine--too much sugar will cause it to "sit" in the belly and not distribute as readily into the blood steam. A good "non-sweet" example would be the Vlasic (brand name) variety, i.e. pickles sold in the refrigerated counters of the store. The "brine" usually takes around 15-20 minutes to get into your system, but it will help retain some minerals and stave off some of the cramping.
Do note that on a hot, humid day, you'll encounter your most serious heat-related problems after
you've come off the court. Since you're no longer exerting yourself physically, your heart rate drops so less blood is pumped nearer to the skin's surface (along with oxygen) which facilitates in the "cooling" process. However, your body has yet to cool down. So, as a result, the body compensates by INCREASING the sweating process (i.e. you can't stop sweating). As a result, your electrolytes can litterally be flushed ("pour") out of your body unchecked and end up in that "puddle' that is now collecting on the ground beneath you. Beware the "NaCl laden" pickle brine remedy if you've got some "cardiac" issues; then again, if you have a shaky ticker, you shouldn't be playing in that kind of heat/humidity in the first place.
Additionally, hydrating begins the day(s) before; thirst alone is no indicator of how much water/fluids you need. Beware caffine and alcohol; diaretics that increase the chance of de-hydration.