Novak Djokovic has called for “clear protocols” and “standardised approaches” to doping cases in tennis, with Jannik Sinner escaping a ban after testing positive for a banned substance.

The world number one tested positive twice in March for clostebol, a steroid that can be used to build muscle mass.

The Italian was cleared of any wrongdoing or negligence by a court last week after the International Tennis Integrity Agency (ITIA) ruled that Sinner had been inadvertently infected with the substance by physiotherapist Giacomo Naldi.

Naldi had applied a spray to a cut on his hand, which he had received from fitness trainer Umberto Ferrara, before treating Sinner.

The 23-year-old, who no longer works with the duo, was given an automatic provisional suspension after testing positive, but was allowed to continue playing after successfully appealing his case.

Some players have suggested that Sinner has been given special treatment. World number two Djokovic also said at the US Open: “I understand that the frustration among the players comes from a lack of consistency.

“As I understood it, his case was approved in principle at the time it was announced. But I think five or six months had passed since the news (of the positive tests) was brought to him and his team.

“So yes, there are many problems in the system,” the 24-time Grand Slam champion added. “We see a lack of standardized and clear protocols. I can understand the feelings of many players who wonder if they are being treated the same.

“Hopefully the governing bodies of our sport can learn from this case and come up with a better approach for the future. I think collective change needs to happen.”

The decision not to punish Sinner after the positive tests has been criticised on social media by a number of players, including Australian Nick Kyrgios, Britain’s Liam Broady and Canadian Denis Shapovalov.

Australian Open champion Sinner is the top seed at the US Open, the final Grand Slam of the year, which begins on Monday.

“Many players – I won’t name any – have had similar or very similar cases where they didn’t have the same outcome,” added Serbia’s Djokovic, 37.

“The question now is whether it is a matter of money, whether a player can afford to pay a significant amount of money for a law firm that can then handle his or her case more efficiently.

“I don’t know. Is that a case or not? That’s something that I really think we need to look at more collectively, to look at the system and understand how these cases don’t happen, not the case itself, but how we can standardize everything so that every player, regardless of their ranking, status or profile, can get the same treatment.”