At such a high level, small changes in diet and lifestyle can tip the balance to one competitor over the others. A Serbian doctor examined Djokovic in the summer of 2010 and told him he was sensitive to gluten and dairy. From then on, Djokovic changed his diet significantly. As he cleaned up his meals and avoided empty calories, what was once the weakest part of his tennis preparation became another strength. His run to the 2010 U.S. Open final made it clear the change was already starting to take effect.