Saturday saw Spurs end their 17-year wait for a win at the home of their arch-rivals when they came from 2-0 down at half-time to triumph 3-2 at the Emirates Stadium.
Defoe said: "Second half, I think they eased off a little bit. Sometimes, it's difficult when you go 2-0 up. People think you just go on and you win the game quite comfortably.
"But sometimes you take your foot off the gas.
"I think they did that and when we got the goal back - the first goal - after that I knew we'd win the game."
Defoe said: "As players, you don't dwell on the past and what's happened before. It's a new day and you always look forward to the games.
"We had a fantastic season last year; we want to try to do the same, just keep getting points and try to get in that top four again."
Defoe's 45-minute run-out was his first since recovering from the ankle surgery that had sidelined him for two months.