The lactic acid builds up, your muscles are screaming, your body temperature sky-rockets -- trying to stay two shots ahead of your opponent proves impossible as your thoughts fragment into near delirium.
Five hours charging around a tennis court can be sheer hell even when you're winning, but if your body is out of balance then it's an even harder challenge.
Sunday's Australian Open finalists Novak Djokovic and Andy Murray once struggled with their fitness, but both have transformed their tennis careers since discovering a diet secret that is battling for acceptance in the world of mainstream sports science.
The benefits of going gluten free have been extolled for years by alternative health practitioners, but they have been brought to wider attention due to the exploits of high-profile stars such as Djokovic, whose wheat intolerance caused the world No. 1 severe breathing problems.
"I knew from my experience that asthma was not the boy's problem but rather it was reflecting a symptom of something that was going on with his digestion," says Dr. Igor Cetojevic, who worked with Djokovic for a year up until his Wimbledon triumph in 2011.
"It was his sensitivity to gluten that was compromising his lungs," he adds, citing that in Chinese medicine there's believed to be a relationship between the large intestines and the lungs.
Our diet has changed dramatically in the past century, especially in Western countries. It's a trend that's spreading around the world due to the globalization of commerce, and the food industry in particular.
"I believe one of the problems is over consumption of gluten-containing grains, especially wheat, and the fact that it is genetically different from ancient wheat," says naturopathic physician Hamish Everard.
Cutting out wheat-based foods such as bread and pasta could be the best thing you ever do, says one of his clients, tennis coach Pete McCraw, who has helped develop top players such as Maria Sharapova and Jelena Jankovic.
"The energy level that you have is something I've never experienced before. You don't ever have that crash, the high and then the low," McCraw told CNN.
"Those periods of the day when you crave carbs or sugar, feeling drowsy at work or in the car or wherever, you don't have that anymore, it's a completely different thing -- it's sustained, you don't have the foods cravings like I used to.
"It's a completely different energy source that your body's operating from. The ability to maintain a lean athletic figure is effortless."
The problem is not just wheat, but also processed dairy and sugars -- all of which we consume more regularly and in higher quantities than ever before.
"The gluten in food produces certain conditions; dairy, especially when produced conventionally, contributes to other factors to the detriment of our health," says Cetojevic, who is trained in both conventional and alternative medicine.
"Generally, milk is for babies. We don't need dairy products as we get older," he adds, and also warned against the intake of processed sugar.
"The change in food production has increased yields to the point of surplus but has not improved the quality of the food we eat. Often it contains traces of pesticides, hormones, preservatives, artificial colors, flavor enhancers and I won't even begin to talk about genetically modified organisms."
About 18 months ago, McCraw started noticing symptoms of extreme lethargy in young players he was working with.
"They were presenting chronic fatigue symptoms, although they weren't diagnosed in a traditional way, in terms of blood markers -- everything would come back reasonably normal," he said.
"Athletes would be complaining of excessive tiredness, even after light sessions. During a normal daily activity it was fine but then as soon as they began to exercise their blood sugar levels would drop significantly, and that causes concentration-behavior issues, (problems with) focus, emotional control.
"By the normal standards these were healthy teenagers and players in their early 20s, but they weren't healthy."
McCraw, who has held top coaching and development roles in his native Australia, New Zealand, Israel and at the Nick Bollettieri academy, works with players from age grade to elite level.
One of his clients had been seeing Everard to treat his celiac disease -- an autoimmune disorder caused by gluten intolerance that attacks the walls of the small intestine and makes it difficult for the body to absorb and process the nutrients that it requires.
Everard says that while only 1% of people are gluten intolerant, more than half of his clients are sensitive to it -- and 93% of them have seen benefits from eliminating gluten from their diet.
The improvements increase even further when processed sugars such as high-fructose corn syrup are taken out of the equation, says McCraw.
You might think bread and pasta are healthy foods, but essentially they break down into sugars like a chocolate bar does -- and this is not an energy source that human bodies evolved with.