"I think I was just relieved after the first round, I'd been worrying for weeks that I'd be coming last," he said.
At the end of the round, Mr Nahu was sitting on 15 points - a single point above Mr Barnes.
"At that point I had a feeling I could still pick him off," he said. "We were eyeballing each other."
During the final round, New Zealand knowledge, Mr Barnes scored nine points bringing his total to 23. When Mr Nahu's turn came, Mr Barnes said he counted his points and was relieved when Mr Nahu ended up a point short.
Since filming in Easter Mr Barnes has been sitting on the results, and is now swotting for his specialist subject in the semifinals: the life and political career of Joseph Stalin.
He said, "I've carefully picked three books and will go through them, seeing what the common things are, and learning a lot of hard Russian names."
When asked if he was more nervous going into the semifinals, he said "Yes, because the stakes are higher ... but no because I know I won't screw up".
"It's just about what you do on the day."
- Mastermind airs on TV One on Sundays, at 7.45pm.