Uniondale - Defending champion Kevin Anderson has withdrawn from the New York Open because of a right elbow injury.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.