Last year 's runner-up Novak Djokovic withdraws from Olympics . The former world number one was unable to defend his Olympic title in London , and the the 26-year-old had to pull out of last week 's tournament in Toronto as well as this week 's event in Cincinnati . Spanish player has already missed the U.S. Open and North American events . Nadal lost to Novak Djokovic in last year 's final in London . Former world number three has a history of knee problems .
Last year 's runner-up Novak Djokovic withdraws from Olympics . Spanish player has already missed the U.S. Open and North American events . Nadal lost to Novak Djokovic in last year 's final in London . Former world number three has a history of knee problems .