By Nathan Ridley Novak Djokovic hilariously donned a bike helmet as he returned to the Italian Open after being hit on the head by a water bottle just hours before. Djokovic, 36, had a nasty surprise on Friday while signing autographs for fans, as an aluminum-type water bottle fell from a rucksack above and bumped him on the head. The 6ft 2in Serbian immediately dropped to the floor, clearly in pain. Security staff rushed in to make sure that Djokovic was okay, as spectators at Centre Court had been leaning over the railings to have their hats and tennis balls signed. But after issuing an upda…