You can't play tennis without tennis balls. Yet an increasingly vocal group of players says that the tennis balls used on tour are behind a major problem: They're causing injuries.

Green tennis balls. Green balls are stage 1 balls. According to the ITF, the intention is for players to progress from using stage 3 (red) balls to stage 1 (green), and then on to the standard ball.

For more than a year now, pro tennis players have not been altogether happy with the physical makeup of the tennis balls used on tour, and the ones being used this year at the BNP Paribas Open ...