But Lynch also used the skills once shared with ex-PGA Championship winner Day to manipulate a girl after allegedly molesting her, a Brisbane court has heard.
Lynch, 68, is accused of grooming a girl before engaging in multiple indecent acts with the "gifted" junior golfer from April 2017 until December 2019.
Lynch exchanged text messages and phone calls with the girl, progressing to "dirty talk" and showing her pornography, crown prosecutor Chris Cook said.
The golf mentor also sent indecent photos of himself and asked for some in return before engaging in physical acts with a "vulnerable young girl", the District Court heard.
Mr Cook said Lynch convinced the girl not to speak to anyone about their "secret relationship" including her own family.
During one alleged incident Lynch also told her that "it would help her golf so much", he said.
"He is a successful mind coach (with) Jason Day, all these people we have heard about," Mr Cook said.
"He was able to manipulate the mind of (the girl) not to tell her family any of it.
"She didn't say anything because she trusted that everything that Mr Lynch did was in her best interests."
Even when she did confide in someone, the girl was too scared to reveal too much, Mr Cook said.
He cited evidence from a junior golfer who asked the girl if she should hire Lynch as a mind coach in late 2019.
"No, he's a dodgy old man," the girl said in evidence read in court.
"Why?" the junior golfer asked.
"Because he molested me," the girl replied.
The girl later told police she didn't want to tell the junior golfer too much "because I was just scared".
The girl stayed at Lynch's house on and off for 12 months from March 2018, the court heard.
But she reduced her contact from May 2019, even blocking Lynch's number on her phone after she "saw something on telly".
"They (TV show) were talking about people being groomed," the girl said in evidence read to the court.
"That's the first time I really sort of thought ... that sounds exactly like what Sean was doing to me - I cut off all contact."
Defence barrister Tony Kimmins said some charges were based on the girl's generalisations, taking aim at her "foggy" memory.
The girl's police interview "was all over the place, it was very difficult to actually follow".
Mr Kimmins said the girl also looked like she was trying to think of something else to say in the police interview shown to the jury.
"On a number of occasions she has a half smile on her face (and) stops as if she is in some sort of trance," he said.
"It's not as if a person is remembering what occurred."
Lynch has pleaded not guilty to 11 charges including indecent treatment of a child under 16, possessing child exploitation material and grooming.
The trial before Judge Vicki Loury continues.
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