Grass was the US Open’s surface from 1881 through 1974.
The tournament began as the U.S. National Championships in 1881, when its early competitions were played on grass. Grass remained the surface for nearly a century, including the event’s years at the West Side Tennis Club in Forest Hills, Queens.
The tournament changed to Har-Tru clay in 1975, partly because organizers wanted a surface with different playing characteristics and better television appeal. In 1978, it moved to the newly developed USTA National Tennis Center in Flushing Meadows and adopted hard courts, which remain in use today.
A frequent mistake is to remember clay because the US Open used it for three editions, from 1975 through 1977. The longer historical period, however, ended with grass in 1974. This also explains why Chris Evert’s US Open titles span two surfaces: clay in 1975–1977 and hard court from 1978 onward.