The US Open used clay courts from 1975 through 1977.
The surface was a green-colored Har-Tru clay, installed at the West Side Tennis Club in Forest Hills, Queens. Har-Tru is generally faster and harder than traditional red clay, with different bounce and movement characteristics. The switch ended the tournament’s long run on grass.
Organizers changed surfaces partly because players complained about the deteriorating grass courts and uneven ball bounce. The clay years were also intended to create a more television-friendly event, while added floodlights made evening matches possible.
The 1977 tournament was the final US Open at Forest Hills. In 1978, the championship moved to the newly built National Tennis Center in Flushing Meadows and changed again, this time to hardcourt. Jimmy Connors remains the only player to win US Open singles titles on grass, clay, and hardcourt.