"To be honest, I was quite surprised," admitted Defoe, who missed two games with a sore calf, on Pompey's website.
"When I got the ball my intention was to cut inside, but for some reason I decided to go the other way. It was one-on-one and it was nice to see it go in.
"It was good to come back and score.
"The lads were fantastic against AC Milan in the week, so I just wanted to come in and do a job to help the team get three points."