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More than six years ago, Novak Djokovic's physical conditioning was questioned after he began to cramp up on court or suffer from fatigue towards the end of matches. Now the Serb is the dominant force in men's tennis.

So what's changed? His former doctor, Igor Cetojevic, says food was at the heart of the problem and says too many players mistakenly rely on a diet of croissants, pizzas and coffees to keep them going for up to five hours on a tennis court. Cetojevic says it's all about getting "the right petrol in the engine".

Photo: Novak Djokovic eating a banana. Credit: Matthew Stockman/Getty Images.