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Equipped with a tennis racket-like mesh center, researchers at the Swiss Federal Institute of Technology were able to build quadrocopters that could be controlled so accurately, they could play a game of tennis

The robots, based on Ascending Technologies' "Hummingbird" quadrocopter (a aircraft lifted off the ground by four rotors), were programmed to return a ball tossed their way by angling their bodies to allow the ball to bounce off the mesh. The coolest part, though, is when the two robots enter into a tennis rally, bouncing the ball back and forth between each other. 

Impressive, yes, but we want to see Federer go up against one of these. 

[via Wired UK