World number one Jannik Sinner has been "worn down physically and mentally" by an investigation into his positive tests for a banned substance, says his coach Darren Cahill.

In March, Australian Open champion Sinner twice tested positive for clostebol – a banned substance that can be used to build muscle mass.

However, an independent tribunal last week found he was inadvertently given the substance in an over-the-counter spray by his physiotherapist Giacomo Naldi.

Sinner has won ATP-level titles at Halle and Cincinnati since the positive tests, but he was eliminated from the French Open at the semi-final stage and only reached the last eight at Wimbledon.

He then missed the Paris Olympic Games after being diagnosed with tonsillitis, and Cahill feels it was no coincidence that he failed to hit top form while the probe was ongoing.

"I guess if you are not a Jannik fan, you may not see too much difference in what he has been doing the last few months because he has still been playing quite well," Cahill told ESPN.

"But if you are a Jannik fan, you would have seen a big change in his body language, his physicality on the court, his excitement to be on the court.

"He's struggled and I think it's worn him down physically and mentally. He got tonsillitis, which is the reason why he missed the Olympics."

Some have been critical of the decision to clear Sinner of any wrongdoing, with 2022 Wimbledon runner-up Nick Kyrgios particularly vocal in his reaction.

Kyrgios wrote on X it was "ridiculous" that Sinner had escaped a ban, saying the Italian deserved to be hit with a two-year suspension.

Former Wimbledon semi-finalist Denis Shapovalov also suggested the decision was at odds with other players receiving bans over similar incidents.

Cahill, however, is of the belief Sinner would never intentionally take a banned substance, adding: "I just want to stress that he's maybe the most professional young man that I have ever had the chance to work with. 

"He would never ever intentionally do anything, and he's in a situation that is incredibly unfortunate."