Under the sun of the French Open tennis tournament at Roland-Garros in Paris, tennis players do not only fight against their opponent. When temperatures exceed 30°C, sometimes reaching 35°C, the heat itself becomes a real physiological challenge.
This year, during the Grand Slam tournament on clay, the Norwegian Casper Ruud himself described having “feeling like a zombie” during his first round against the Russian Roman Safiullin. Victim of a severe heat stroke, he explained after the match that he had dizziness, difficulty seeing the ball and the feeling that his body was no longer able to cool down properly. After losing eleven consecutive games and conceding a set 6-0, the one who is one of the best players in the world finally managed to regain control of the match after almost four hours of effort, in a temperature above 30°C.
In the first round of Roland-Garros 2026, four withdrawals were recorded, while the tournament generally has many fewer. Behind these spectacular images lies a complex physiological reality: during intense exercise in high heat, the body constantly fights against hyperthermia, that is to say the excessive increase in body temperature.
When the body tries to avoid overheating
Human beings must maintain their internal temperature around 37°C in order to ensure the proper functioning of their organs, particularly the brain. However, during intense physical effort, the muscles produce a lot of heat. Thus, even in so-called “normal” temperature conditions, body temperature increases during intense exercise and can reach 38.5 to 39°C in trained athletes.
But when the outside temperature rises sharply, dissipating this heat becomes much more difficult. In a sport like tennis, the problem is accentuated by direct exposure to the sun, the heat accumulated by the clay court and the poor air circulation on the court.
From around 39°C core temperature, physical and cognitive performance generally begins to decrease significantly. When the temperature continues to increase, the body gradually enters a zone of significant thermal stress likely to cause discomfort or, in the most severe situations, true exertional heat stroke, a medical emergency characterized by severe hyperthermia associated with neurological disorders.
Sweating: the “physiological cost” of our primary cooling system
To avoid this overheating, the body sets up several closely linked defense mechanisms. The main cooling system is perspiration. Contrary to popular belief, it is not sweat itself that cools the body, but its evaporation. When water changes from a liquid to a gas on the surface of the skin, this transformation absorbs thermal energy from the body, thereby cooling it.
To make this system more efficient, the body also increases blood flow to the skin through cutaneous vasodilation. A significant part of the blood is thus redirected towards the superficial areas in order to promote heat exchange and the evaporation of sweat.
But this redistribution of blood flow has a significant physiological cost. A portion of the blood flow available to active muscles is mobilized for skin cooling. This competition between cooling the body and nourishing the muscles contributes to reducing exercise capacity and contributes in particular to reducing the maximum quantity of oxygen that the body can use over a given time (or VO2 max) under high heat.
Sweating also causes significant losses of water and sodium. During a match in high heat, a player can lose several liters of sweat per hour. Even moderate dehydration – around 2% of body weight – is already enough to reduce physical and cognitive abilities.
Why tennis is particularly vulnerable to heat
Not all sports react in the same way to high temperatures. Tennis combines several characteristics that make this effort particularly difficult. First, matches can last a very long time, sometimes more than four or five hours during Grand Slam tournaments. Then, the game constantly alternates between explosive accelerations, changes of direction, powerful strikes and relatively short recovery times.
But heat doesn’t just disrupt muscles. The brain is also strongly affected. Studies show that dehydration and an increase in body temperature gradually impair concentration abilities, reaction time and decision-making. The perception of effort also increases significantly. In other words, at the same intensity, the effort seems much more difficult in high heat.
In a sport as technical and tactical as tennis, the loss of lucidity can be enough to change a match. Players must make thousands of microdecisions, some of which can directly influence the outcome of a set or game.
The brain also acts as a real protection system. When the body temperature becomes too high, it gradually reduces the intensity of the effort in order to limit the risk of overheating. This reduction in performance can appear even before the muscles are completely exhausted.
In a sport as technical and tactical as tennis, this drop in lucidity can be enough to change a match. Over several hours of play, players must make thousands of micro-decisions – placement, trajectory, power or tactical choices – some of which can directly influence the outcome of a set or a game.
Why professional players resist better
Professional players develop numerous adaptations over the years that allow them to better tolerate heat. One of the most important is thermal acclimatization. After several days of training in high heat, the first adaptations already appear: sweating begins more quickly, the heart rate increases less sharply and the body temperature becomes more stable for the same effort. After several weeks of repeated exposure, these adaptations become more marked and more lasting.
The volume of sweat produced also increases, which improves cooling capabilities. At the same time, the sweat glands become more efficient at reabsorbing sodium before its elimination from the surface of the skin. Acclimatized athletes therefore lose proportionally less sodium despite often sweating more.
The preparatory tournaments on clay played in Madrid or Rome took place in relatively cool conditions, which surely limited prior acclimatization before arriving in Paris.
Blood volume also increases, thus improving cardiovascular stability, thermoregulation capabilities and the maintenance of oxygen supplies to the muscles. These adaptations are particularly worked on before certain tournaments like the Australian Open, where extreme temperatures are frequent.
But this year, the conditions seem to have surprised more players during the first week of the Roland-Garros tournament. The preparatory clay court tournaments played in Madrid or Rome took place in relatively cool conditions, which probably limited prior acclimatization before arriving in Paris. This more brutal climate variability could become more and more frequent with climate change.
What advice for amateur players?
These issues do not only concern professional players. Every summer, thousands of amateur tennis players also compete in very hot weather.
Among amateur athletes, it is essential to convey a simple message: abandoning a match is infinitely preferable to continuing an effort that would potentially endanger their health.
Among amateurs, what measures can be taken to limit the risks?
- Hydration must start before the match: waiting until you are thirsty is already a bad signal, because dehydration is often already set in. During prolonged exercise, drinks containing sodium can help compensate for losses linked to sweating.
- Acclimatization also plays a major role: it is important not only to gradually expose oneself to heat, but also to gradually train in these conditions in order to improve the body’s thermoregulatory capabilities.
- During the match, it is important to take advantage of breaks to cool down: damp towel, shaded areas, ice around the neck or head can help limit the increase in body temperature.
- Certain symptoms should alert you immediately: dizziness, nausea, confusion, chills, headaches or stopping sweating. They can be a warning of exertional heatstroke, a potentially serious medical emergency.
A sporting issue… but also human and climatic
With climate change, extreme heat events are becoming more frequent and more intense in many regions of the world. Major sporting competitions will increasingly have to integrate this new reality. At Roland-Garros, the heat no longer represents just discomfort. It becomes a major factor in performance, recovery, safety and sometimes even “physiological survival” on the court.
But these extreme conditions also reveal something fascinating about high-level sport: the ability of athletes to continue to make decisions, manage their emotions and maintain their physical commitment despite major physiological constraints. In matches where each point can change the outcome of the match, managing to maintain your lucidity in high heat becomes a performance in itself.
Perhaps this is also why certain victories at Roland-Garros seem to provoke such intense emotions: beyond the opponent, players have sometimes just won a fight against their own physiological limits.
