A muay thai instructor is accused of sexually assaulting a woman whom he was "massaging" after she attended a trial class at the gym he co-founded.

On the pretext of offering the 33-year-old woman a free "Thai massage", Tan Wai Luen got her to strip to her underwear and lie on a massage table where he sexually assaulted her with his finger, prosecution told a district court yesterday, at the close of Tan's trial.

Tan, 30, faces one charge of sexual assault by penetration. He has flatly denied sexually assaulting the woman throughout the trial.

The woman, who cannot be named to protect her identity, had testified to attending a free trial class at Encore Muay Thai on Oct 1, 2016, after she saw an advertisement on Facebook.

Tan was the instructor for her class, which was attended by three other women whom she did not know.

The woman was left alone with Tan after the class when the other three left, and he sat her down to discuss gym packages, said Deputy Public Prosecutor (DPP) Kavita Uthrapathy in her closing submissions.

At some point during the discussion, Tan offered the victim a massage, and told her that he was the only one trained to do massages there, said DPP Kavita. The woman accepted.

While he was massaging her inner thigh area, Tan slipped his finger into her panties and sexually assaulted her, the DPP added. "(He) took it out immediately when she turned around to look at him and shouted 'oi'.

"The accused did not apologise or say anything to her," said DPP Kavita.

The massage then continued for about 10 minutes, and the woman left the gym thereafter.

The woman contacted a co-founder of the gym the next day about the incident, and the latter lodged a police report.

Tan conceded at the start of the trial to accidentally touching the woman's genitals during the massage, the prosecutor said. He later went back on this and said "it was impossible" that he had touched her there at all, as he had "stopped performing the massage on her at her lower thigh area".

DPP Kavita said both did not know each other prior to the incident, and there is no motive for the victim to make false allegations. A verdict is expected next year.