The 84-year-old artist -- a fixture on British screens for decades -- said the teenager had been "flirtatious and coquettish" with him and seemed to give him an "invitation".
Taking the witness stand at Southwark Crown Court, Harris sang part of his hit "Jake the Peg" and described his wobble board trademark sound, which he accidentally invented by shaking a hot painting board.
Harris is accused of 12 counts of indecent assault against four female complainants, aged from seven or eight to 19, between 1968 and 1986.
Seven of the counts relate to his daughter's friend, who claims Harris assaulted her over several years, dating back to when she was 13 and coming out of a shower during a trip to Hawaii.
But the television presenter insisted their relationship only began when she was an adult.
Harris said he felt "sickened" by his own behaviour, blaming himself for alcohol problems the woman later developed.