Speaking to reporters as he prepares to defend his US Open title from Monday, reigning champion Novak Djokovic said players who have complained about the lack of transparency in Sinner's case have a valid point.

"I understand the frustration of the players because of the lack of consistency," said the most successful Grand Slam winner in history (24 titles).

"As I understand it, his case was resolved the moment it was essentially announced. But, you know, I think five or six months passed from the time the news (of the positive tests) was communicated to him and his team."

"So, yes, there are a lot of problems in the system," Djokovic said. "We see a lack of clear and standardised protocols. I can understand the feelings of many players who wonder if they are being treated in the same way."

Several players have suggested that Sinner received special treatment because of his ranking and questioned why he was not provisionally suspended while the investigation that cleared him took place.

However, Sinner insisted on Friday that he was treated the same as any other player. "There are no shortcuts, there is no different treatment, they all follow the same process."

The 23-year-old Italian was informed earlier this year that he had tested positive for low levels of the banned anabolic steroid clostebol during the Indian Wells Masters in March, as well as in another out-of-competition test eight days later.

Sinner revealed that he had been provisionally suspended for several days, although this was not made public at the time.

He and his team quickly appealed, arguing that the drug entered his system when his physiotherapist used a spray containing it to treat a cut and then gave him massage and sports therapy.

He was allowed to continue playing while investigators looked into the case, and the International Tennis Integrity Agency announced on Tuesday that he had been cleared of any wrongdoing, meaning the current world number one has not been suspended.

Another of today's top players and world number three, Carlos Alcaraz, suggested on Saturday that he believed there was more to the case than had been made public but refused to criticise the ITIA's decision.

"You know, I'm pretty sure there are a lot of things we don't know, you know, within the team or within everything," Alcaraz said. "But if they allowed Jannik to continue playing, it's for a reason. They said he was innocent, that's all I know and that's all I can talk about," concluded the Spaniard, who will play against Li Tu at the Arthur Ashe Stadium on Monday.

Djokovic said the case was a clear example of why the sport needs the controversial Professional Tennis Players Association, which he helped found to give players a greater voice in the sport.

He wondered if some of these cases, like Sinner's, could be resolved with little publicity because the player has the resources to hire expensive and effective legal representation, while other players do not.

"I don't know," Djokovic said. "Is that the case or not? That's something I really think we need to look at more together, to look at the system and understand how these cases don't happen - not the case itself, but how we can standardise everything so that every player, regardless of their ranking, status or profile, gets the same kind of treatment," concluded the Serb, who will defend last year's title against Albot at the start of the final Grand Slam of the year.