http://www.bbc.com/sport/0/tennis/29089164Caroline Wozniacki reached her second US Open final after China's Peng Shuai had to retire in stifling conditions.
The Dane was leading 7-6 (7-1) 4-3 when Peng began holding her leg in pain, and was then helped off the court for medical attention. Peng returned 10 minutes later but could complete only five points before collapsing once again in the heat.
Signs of heat stroke - Mayo Clinic offers a few pages of information on the subject. You don't have to be an athlete to have a heat related illness.
http://www.mayoclinic.org/diseases-cond ... n-20032814
Heatstroke is a condition caused by your body overheating, usually as a result of prolonged exposure to or physical exertion in high temperatures. This most serious form of heat injury, heatstroke can occur if your body temperature rises to 104 F (40 C) or higher.
Heatstroke requires emergency treatment. Untreated heatstroke can quickly damage your brain, heart, kidneys and muscles. The damage worsens the longer treatment is delayed, increasing your risk of serious complications or death.
Heatstroke symptoms include:
High body temperature. A body temperature of 104 F (40 C) or higher is the main sign of heatstroke.
Altered mental state or behavior. Confusion, agitation, slurred speech, irritability, delirium, seizures and coma can all result from heatstroke.
Alteration in sweating. In heatstroke brought on by hot weather, your skin will feel hot and dry to the touch. However, in heatstroke brought on by strenuous exercise, your skin may feel moist.
Nausea and vomiting. You may feel sick to your stomach or vomit.
Flushed skin. Your skin may turn red as your body temperature increases.
Rapid breathing. Your breathing may become rapid and shallow.
Racing heart rate. Your pulse may significantly increase because heat stress places a tremendous burden on your heart to help cool your body.
Headache. Your head may throb.