"His and hers!" tweeted @AnaIvanovic today referring to the two tennis balls sitting on her racquet.
Wilson, the "Official Ball of the US Open," confirms. The main difference is the felt used.
Pro tour wide, ATP men use the equivalent of a Wilson Extra Duty ball and WTA women, the Regular Duty, a tennis ball more generally used for clay court play.
According to the USTA, "This selection was designed to increase the ball speed in women’s game and to slow down the velocity of the balls in the men’s game." Thicker Extra Duty balls travel a little more slowly than their trimmed down Regular duty relative.
I wonder what's used for mixed dubs?