Prominent Games

Tennis for two (1958)
Spacewar! (1961)
Pong (1972)

One of the simplest and earliest examples of game UI is Tennis for Two (1958), in which players manipulate the movement of a ball over a simplified tennis court. The only UI involved is where the player presses a button to hit the ball and adjusting the angle with a control knob. The game continues until the player misses.

Spacewar! (1961) involved two players, where each player controls their own spaceship to fight each other. While the gameplay is more complex than Tennis for Two, it still involved mostly reflexes and muscle memory over time.

Pong (1972) involved two players battling each other with passdles onscreen to return the ball to the opponent's side without missing. If the player fails to hit the ball, the other player gets a point. This was a direct segway for game developers to add a point system to their games for competitive play.