Novak Djokovic apologised for his US Open disqualification on Sunday, declaring himself "extremely sorry" for the moment of madness which saw him defaulted from the tournament. In a statement on Instagram just hours after his sensational exit in New York, the Serbian world number one said he had been left "really sad and empty" by the controversy. The 33-year-old was tossed from the tournament after accidentally hitting a tennis ball into the throat of a line official during his fourth round match with Spain's Pablo Carreno Busta. "This whole situation has left me really sad and empty," Djokovic said. "I checked on the lines person and the tournament told me that thank God she is feeling ok. "I'm extremely sorry to have caused her such stress." rcw/bb