Serbian superstar Novak Djokovic called for "clear protocols" and "standardized approaches" to tennis doping cases on Saturday in response to the announcement that top-ranked Jannik Sinner had escaped a ban despite two positive tests this year.

Djokovic, speaking to reporters as he prepared to defend his U.S. Open title, said the players who bemoaned a lack of transparency in the Sinner case make a valid point.

"I understand the frustration of the players is there because of lack of consistency," the 24-time Grand Slam champion said.