Defoe's build up to the World Cup has seen him score four goals in 23 league matches for Bournemouth this season but the veteran says he is not worried as his past exploits prove that he can deliver when called upon.
"I got a letter. All the players that are in contention get a letter to tell you the schedule for the summer," Defoe told ESPN.
"I don't feel like I have anything to prove. I'm quite relaxed about it to be honest. The manager knows what I'm about and, if I'm called on, I'm ready to go to another tournament."
Defoe believes his case for World Cup selection would have been helped if he had been given more game time.
"I feel like if I'd played more games, we'd be sitting here and it'd be a different conversation. Throughout my career whenever I've played and got rhythm, that's when I get my goals. And I've proved it," Defoe added.
"Against Lithuania at Wembley, I had about four touches and scored. Did anyone see that coming? No...
"I've never really thought about retiring from international football because I still feel sharp, I still feel in my heart I can score goals at that level."
Defoe has represented England in disappointing campaigns at the 2010 World Cup and the 2012 European Championship and knows the criticism the players face from a success-hungry British media.
The frontman, however, says he is baffled by the media's jabs aimed at current players Dele Alli and Raheem Sterling.
"Everyone knows that it's different in other countries. It would make a massive difference if the press got behind us more. Instead, when things go wrong, you get players getting crucified..." he said.
Southgate has until June 4 to name his World Cup squad, ahead of England's opener against Tunisia on June 18.
(Reporting by Aditi Prakash in Bengaluru; Editing by Toby Davis)
