Harris, 84, who is accused of abusing the woman for her entire teenage life, said she flirted with him and invited his sexual advances when staying at his home in southern England.
"It seemed to me that she was being very flirtatious with me," he said. "As you can imagine it was a very flattering thing for this young lady to suddenly be showing an interest in me.
"I can remember my heart was thumping away," he said from the witness stand at Southwark Crown Court as he gave evidence for the first time in his indecent assault trial, now in its fourth week.
A mainstay of family entertainment for more than 50 years, Harris is charged with 12 counts of indecent assault against four girls, some as young as seven or eight at the time of the offence, between 1968 and 1986.
He denies all the charges.
Harris is the biggest name to go on trial since British police launched a major investigation after revelations that the late BBC TV host Jimmy Savile was a prolific child sex abuser, leading to the arrest of more than a dozen ageing celebrities.
The court has previously heard that he first assaulted the woman when she got out of the shower aged 13. He then repeatedly abused her until she was 28 years old, the jury of six men and six women was told.
Describing another incident at his home, Harris said: "She seemed very very flirtatious and coquettish - looking at me with some sort of sexual chemistry."
Sonia Woodley, defending, asked: "Would you have touched her ... if you had thought she didn't want you to?"
"No. It seemed to be an invitation," he replied.
Wearing a dark, pinstriped suit, pink shirt and purple tie, Harris told the jury he found it hard to discuss his affair.
"It's highly embarrassing ... I was a married man and I shouldn't have been doing it," he said as his wife Alwen watched on and his agent blew a kiss from the public gallery.
The entertainer, known to millions in Britain and Australia for his work as a television presenter and a string of pop hits, earlier told the jury a series of rambling anecdotes detailing his rise to stardom and even sang a snatch of his famous comedy song "Jake the Peg".
He demonstrated the sounds of a didgeridoo and mimed out his invention of the wobble-board, an instrument he made famous in the 1960s.
Asked whether he had ever hugged the woman as a young girl or made comments about her appearance, he said: "Yes I have. I'm a very touchy-feely sort of person.
"My policy in life has always been to compliment people if they look good. I've been told to stop doing it because people in this day and age quite often take offence."
The trial continues.