The Stadium of Light erupted when Defoe blasted a spectacular left-footed volley past Tim Krul in the 45th minute.
Defoe revealed that as soon as he connected with the shot, he knew it was going in.
"As the ball was dropping, I was just concentrating on the strike," he told Sky Sports. "During the week I've done a lot of shooting, and as soon as it left my foot I knew it was in. It was an unbelievable feeling. I was very emotional for some reason, as everyone wanted the three points so badly.
"I tried to blank it out, because if you think about the pressure too much you get tense and can't perform.
"The lads have been telling me what today would be like with the atmosphere, and it was amazing. We've beaten them for a fifth time in a row and what a feeling.
"Hopefully the fans can be happy and we can kick on."
Thanks to Defoe's goal, Sunderland currently sit in 15th place on the standings with 29 points.