A muay thai instructor offered Thai massage to one of his female trainees even though he was not trained to perform it.

He then sexually assaulted the woman while massaging her.

Tan Wai Luen, 30, was found guilty of sexually penetrating the 33-year-old woman without her consent on Oct 1, 2016.

He is one of the co-founders of Encore Muay Thai in Joo Chiat Road but is no longer an instructor there.

In delivering her verdict yesterday, District Judge Ong Chin Rhu said that the victim's testimony in court was credible.

The judge said Tan "lacked credibility" and his version of events was "contrived".

The victim, who cannot be named to protect her identity, testified that she attended a free trial class at Encore Muay Thai after she saw a Facebook advertisement.

Tan was the instructor in the class, which had three other women whom she did not know.

Deputy Public Prosecutor Kavita Uthrapathy, in her submissions, told Judge Ong that the woman was alone with Tan after the lesson when the other three left and he sat her down to discuss gym packages.

The DPP said: "At some point during the discussion, the accused offered the victim a free Thai massage and told her that he was the only one at the gym who was trained to do Thai massages.

"The victim accepted the offer of a free Thai massage and, as directed by the accused, proceeded to the area in the gym with a massage table, closed the curtains and took off her clothes, except for her panties, lay face down on the massage table and covered herself with a towel."

The court heard that Tan started massaging the woman before he sexually penetrated her.

She shouted at him and the massage continued for about 15 minutes after that.

The woman was about to leave the gym after dressing when she noticed that its main door was locked from the inside. She unlocked the door by turning a knob and left.

"That was when it dawned upon her that the entire ordeal was premeditated and the accused had deliberately locked the main door," said the DPP.

The next day, the woman told another co-founder of the gym about her ordeal and the latter lodged a police report.

The DPP said that Tan initially denied giving the woman a massage, in his statement to the police, but later admitted that he had done so, during the trial.

Tan will be sentenced on Feb 7.