The crumbly, white cheese sells for 1000 euros ($1233) a kilogram, but it's unclear exactly how much the Australian Open winner has bought. Djokovic is behind a new chain of restaurants set to open in his home country and he wanted to shore up enough to guarantee continued supply. Pule has just one producer, a farm about an hour west of Belgrade, and each kilogram requires 25 litres of donkey milk. The farm's manager, Slobodan Simic, said the secret to the taste was that the donkeys were raised on one of the country's famous wildlife and nature reserves. "It will save a lot of effort having to deal with various restaurants, with only one customer buying the lot we don't have to worry too much about salesmen," Mr Simic told the UK Daily Telegraph of Djokovic's extraordinary purchase.

“It is a great vote of confidence as well, in what we do here." Reuters reported last month that the farm was home to 130 Balkan donkeys. The females in the herd are milked three times a day. "With these animals - aside from the fact that we are saving them from extinction, as their numbers are at their lowest possible in these areas - we are making certain products which will help their survival, their breeding and acceptance by people," Mr Simic told Reuters. "No one is producing it in Europe or the world, there's no such information and no one would produce it because it would be excessively expensive. "We dared to make such a product, offered at an acceptable price of about a thousand euros, and that makes it the most expensive cheese in the world. There is no cheese as expensive at the moment, nor more valuable, and in such low quantities, as ours."

According to ancient Egyptian folklore, Queen Cleopatra bathed in donkey's milk, but if that routine was repeated using pule, it would cost about 5000 euros ($6168) a wash, Mr Simic said.