Warming up ahead of his US Open title defence, world number two ranked tennis player Novak Djokovic has been stopped in his tracks by a very unusual marriage proposal.

Running through a series of service drills on one of the Flushing Meadows courts, where the US Open begins tomorrow, Djokovic was interrupted by a young boy's voice.

"Djokovic... will you marry me?" the boy yelled from the stands where loyal fans were gathered to watch the session.

The defending champion walked over to help the boy onto the court, where he invited him to take his place at the baseline.

The boy, who was wearing tennis apparel, proceeded to impress with a series of booming serves, before leaving the court to the cheers of the crowd.