Uniondale - Defending champion Kevin Anderson has withdrawn from the New York Open because of a right elbow injury.
Frances Tiafoe, the American who beat Anderson en route to the Australian Open quarter-finals, was added to the field on Thursday.
Anderson, the world No 5, was hurt during his second-round defeat to Tiafoe at last month's Australian Open.
Anderson was the runner-up last year at Wimbledon and said his victory on Long Island "kick-started my year in 2018 and I was very excited to return."
The South African's withdrawal leaves American John Isner as the highest-ranked player in the field at No 9.
Anderson outlasted Isner in a marathon semi-final at Wimbledon last year.
Australian Alex de Minaur, last year's ATP top rookie, also pulled out of the tournament because of an injury.
The tournament starts on Sunday.
I’m really disappointed to not be going back to the @NewYorkOpen to defend my title. I have great memories from last year and will miss the great fans, but I need to focus on getting my elbow back to its best. I wish the tournament all the best and hope to be back next year. ??— Kevin Anderson (@KAndersonATP) February 8, 2019