Every time I see Kenin hitting a double-fault - she hit two in that game - I wonder why her coach (dad) never got her out of that ball toss routine where she looks down after tossing the ball, even if it is only for a second. Bad habits are tough to unlearn and can often show up in big moments. The serve in tennis, much like a golf swing or a cricket batter's trigger movement, is highly complex. Any number of things can break down.