World No.3 Justin Thomas said he was deeply apologetic after using a homophobic slur during the third round of the Tournament of Champions in Hawaii.

Thomas, the defending champion at the $US6.7 million event, missed a five-foot putt on the fourth hole after which he uttered the slur which was picked up by a television microphone.

"There's just no excuse," the 27-year-old American told Golf Channel.

"There's absolutely no reason for me to say anything like that. It's terrible. It's not the kind of person that I am."

"I need to do better. I deeply apologise to anyone and everybody who I offended and I'll be better because of it."

Thomas fired an eagle and six birdies on Saturday for a five-under 68.

He heads into the final final round in a tie for fifth place at 17 under, four shots behind leaders Ryan Palmer and Harris English.