Jack Sock has withdrawn from Atlanta with an ankle injury.
The world No. 14 would have been the second seed at the ATP 250. Sock attributed the withdrawal "to an injury I sustained in practice leading up to the tournament."
It was going to be his first appearance since winning the men's doubles title at Wimbledon, which he suggested could help improve his lackluster singles season.
The withdrawal includes both singles and doubles. Mike Bryan will instead play with Frances Tiafoe.