Irishman Colin Niland was forced to retire with illness during his first round US Open match against Novak Djokovic.
Niland trailed the world number one 6-0 5-1 when he decided that he was no longer able to continue participating.
Serbian Djokovic will now play either Spain's Pere Riba or Argentinian Carlos Berlocg in the second round at Flushing Meadows.
The 24-year-old is trying to win his third Grand Slam of 2011 after victories at the Australian Open and Wimbledon.
