A tennis ball flew across the net. On the other side, a humanoid robot crouched, shuffled sideways in a series of quick steps, adjusted its position and swung its racquet to send the ball back. The robot traded forehands and backhands with human players during singles and mixed-doubles demonstrations. At one point, while chasing a ball, it lost its balance and toppled backwards, drawing gasps from the crowd. It then rolled over, pushed itself back to its feet and resumed playing. The robot was...