I also warned that some spectators, many of whom are far less fit and most of whom are older than the players, may also be at risk, even if not exercising and even if they are well hydrated.
But I would like to add that there is also a theoretical risk that pregnant women may be putting their unborn babies at risk, both in the womb and after birth, perhaps lifelong.
This hypothesis is plausible if the link between season of conception and foetal deaths is confirmed to be in part due to heat exposure. While that is currently uncertain there are suggestive data in support, such as an econometric paper. This found that additional days above 80°F (26.7°C) causes a "large decline in birth rates approximately 8 to 10 months later". The authors comment that air conditioning could be used to substantially offset the fertility costs of climate change, a suggestion which is unfortunately as yet too expensive for most of the world's people.
Pregnant women watching events such as the Australian Open, especially in very hot days and even more so if they are sitting in the sun, may therefore risk having a miscarriage.
However, that is not the only risk. If we hypothesise that a certain "dose" of heat, a metric that combines air temperature, humidity, duration of exposure, clothing and exercise can lead to foetal death, then it follows that a lower dose of heat may cause subtle or even gross foetal abnormality, in infants exposed to a lower dose in utero. This would be analogous to my hypothesis, which I first originated in 2012, that frail, elderly and in other ways vulnerable people who do not die of prolonged heat exposure are likely to suffer from other effects, some of which could be long-lasting, especially if they accidentally fall during a period of recovery.
Furthermore, if there is a threshold of a heat "dose" then it is highly unlikely that one size fits all. The dangerous dose is likely to be influenced by many other factors: age, fitness, weight, other indicators of health status, genes, and perhaps, even month of birth. It is also likely that some stages of pregnancy are more vulnerable than others.
It has long been known that being born in autumn confers a higher life expectancy than being born in spring (at least in non-equatorial regions that have distinct seasons). Could heat in the first trimester be an under-recognised factor helping to explain this?
Of course, this is only speculation. It would require a great deal of study to be more sure. But, since 2012, research funds to study this and similar questions have been virtually unavailable in Australia.
My recommendation to the organisers of the Australian Open is to hold the tournament in a later month. But that suggestion won't be taken seriously for many more years. For spectators who are pregnant or possibly pregnant - I suggest that they sit in the shade and drink lots of water. Wear a hat and bring a fan. Or perhaps consider staying at home if the temperature is too high. Yet, no one knows what temperature is too high. I suggest the risk is likely to be higher with higher temperatures and the relationship is likely to be non-linear, i.e. the additional risk between 43 °C and 42°C is likely to be more than between 21°C and 20°C. And, please, lobby a politician to treat global warming and climate disruption with the seriousness it deserves.