The 84-year-old conceded that when he complimented the teenager on her bikini during the holiday in the 1970s, he was actually referring to her body.
Harris, who was cross-examined by prosecutor Sasha Wass QC during his second day in the witness box at his indecent assault trial, also admitted having a "darker side" alongside his public persona.
The star is accused of 12 counts of indecent assault on four alleged victims between 1968 and 1986, all of which he denies.
Seven relate to the friend of his daughter, who claims he abused her from the age of 13.
Harris sat in the witness box as he admitted that he might have complimented the girl's bikini on holiday when she was 13.
Ms Wass put to him that bringing attention to her bikini was actually a comment on her body, and suggested he was admiring her sexually.
She asked: "Do you accept that when a man tells a woman or a girl that they look lovely in a bikini, they are not actually admiring the clothing, but they are admiring the person's body?"
Harris replied: "Possibly."
Ms Wass said: "And you made it plain to (the alleged victim) on that holiday that you did admire her, admire her body?"
Harris answered: "On one occasion, possibly."
The prosecutor added: "And you admired her sexually, that's what that is."
Harris answered: "It did not compute to that in my mind... in hindsight, I suppose it is."
Earlier, Harris said he knew that touching a 13-year-old was a criminal offence, but denied anything had happened.
As she began cross-examining the veteran star - who described his guilt yesterday at "betraying everyone" with the affair - Ms Wass described him as a "polished performer", saying: "You're pretty good Mr Harris aren't you at disguising that dark side of your character", to which he replied: "Yes".
The prosecutor said: "This case is to decide whether underneath your friendly and loveable exterior there's a darker side lurking.
"The issue we really have to fathom from this court is how dark that dark side really is."
She said the sexual encounters with his daughter's friend were not consensual: "This was child abuse, grooming, and you effectively psychologically dominated that girl into womanhood."
Harris, who yesterday revealed a second affair with a woman he and wife Alwen had allowed to live rent-free in an annexe at their home, has admitted having a relationship with his daughter's friend, but insists it started when she was 18.
Today he told the court it was a "flirtatious thing" made up of isolated sexual encounters, and stemmed from "a feeling of love and friendship".
Moving on to the other charges against him relating to other witnesses, Ms Wass added: "There are a series of events in this case where you have taken advantage of the fact that you are a well-loved children's entertainer."
But the artist told the court: "They are all making it up."
Harris was released from the witness box while friend and pantomime producer Paul Elliott gave evidence.
He described Harris as a "consummate professional" who never did anything inappropriate when they worked on the stage together,
Mr Elliott told the court the entertainer and artist starred in many of his productions up and down the country in the 1980s and 90s and he would have been aware if there had ever been any complaints about him.
Asked whether Harris ever did anything to cause him concern when interacting with the child or young adult performers in his shows, Mr Elliott replied: "Absolutely not."
The trial continues.