Keep your eye on the ball is a traditional tennis tenet.

Novak Djokovic says adjusting to a changing ball is a major challenge ahead of the US Open.

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The Western & Southern Open in Cincinnati uses a Penn ball, which some players say produces a livelier bounce than the US Open Wilson ball.

Djokovic, who underwent right elbow surgery after the Australian Open, says adjusting to the changing ball is the major variable players face preparing for Slams.

"Well, in terms of balls, it's not the same as US Open, and it's something that is not that great, obviously," Djokovic told the media in Cincinnati. "On the clay we also have this kind of situation, you know, where Roland Garros is played with one kind of balls and the other tournaments as a leadup to Roland Garros they play with the other balls, and same here.

"So that's something that obviously is different, and you have to kind of deal with it, accept it. The ball is bouncing quite high in Toronto and here, as well. I think when it's very hot and humid, you know, the ball flies through the air quite a lot, so it's not easy to control the ball. "



The 13-time Grand Slam champion says Cincinnati, which is historically one of the faster hard courts on the circuit, delivers a slightly different bounce day and night.

"But at night the conditions are quite different to day, and the ball is a bit slower," Djokovic said. "So the bounce of the ball, as well, is a bit lower which is nicer for play, so to say. But in these kind of conditions, you obviously need to rely a lot on your serve and trying to hold your serve comfortably and hopefully get lucky on the return."

Photo credit: Christopher Levy