Just so you know from the site's FAQ on grounds passes - you can only purchase them on site the day of the session you want to attend.
GROUNDS PASSES
Grounds passes
only become available to
purchase on-site the day of the session once Center Court and Grandstand Courts are sold out for a session and are available in a limited quantity due to site capacity.
A grounds pass is a ticket that allows the holder to enter the Lindner Family Tennis Center and gain access to all public areas of the grounds EXCEPT for Center Court. All other match and practice courts have open (non-assigned) seating that a grounds pass holder is permitted to occupy. In order to sit in Center Court, a regular ticket with a Center Court seat assignment must be purchased.
A grounds pass is not needed in addition to a regular Center Court ticket. Access to all other courts and public areas of the grounds is included with a regular Center Court ticket.