Rafael Nadal's struggle as a young man will change how you watch him play tennis.

Rafael Nadal, widely considered one of the best tennis players to have ever lived, cannot remember what it’s like to play a match without pain.

All consuming pain.

You wouldn’t guess that from watching the world number one win in the fourth round against Nick Kyrgios at the Australian Open.

The best athletes make the game look easy.

But for Nadal, it’s anything but.

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The 32-year-old suffers from a rare birth defect called Kohler’s foot – meaning the navicular bone in his left foot never properly developed. For a young Nadal, it meant his foot was swelling after playing and was always on the verge of fracture.

For many sufferers, not only is competitive sport out of the question, but the condition means they develop a limp or unusual style of walking.

The Spaniard was 17 when he received the diagnosis, which some researchers believe is caused by excessive strain on the foot at a young age.

A tennis racquet was thrust into Nadal’s hand for the first time by his uncle, Toni Nadal, when he was three years old. Recognising a natural talent, Toni Nadal, a former professional tennis player, began to train him.

By eight, Nadal was winning under 12 championships, while also playing football just as competitively. By 12, he was winning Spanish and European titles.

But pain became more recurrent and more debilitating as he developed through adolescence.

When the diagnosis finally came at 17, Nadal was told by doctors he would have to retire his tennis career – before it had even begun.

This was not a conclusion, however, that Nadal or his father Sebastian Nadal, readily accepted. They sought different specialists, until one recommended an insole that would cushion the arch of Nadal’s foot and redistribute weight to parts that were not so damaged.

The treatment appeared to work. At first.

But pain again became unbearable in 2005, this time not only in his foot, but in both his knees. The redistribution of weight had put enormous pressure on his joints.

At 21 years old, Nadal began suffering from patellar tendonitis in both knees, responsible for pain so severe it would cause him to withdraw from several championships over the course of his career.

Over the next 10 years, Nadal would take 25 months off because of injury – including foot, knee, wrist and more recently, hip pain.

Last year, a photo of Nadal’s hand went viral during the French Open, almost as a metaphor for how much this man is willing to endure without giving up.

It's been 14 years since the winner of 17 Grand Slam titles has played a match of tennis without painkillers.

As we watch him hitting the ball faster, harder and with more precision than just about anyone else, it's worth remembering that despite how easy it looks, for Nadal, every second hurts.

But for him, it's always been worth it.