Novak Djokovic is officially the oldest world No. 1 in ATP history.

At 36 years and 321 days old, Djokovic surpassed Roger Federer's record as he continues to establish himself as the greatest player in the history of men's tennis. Djokovic is set to begin his 420th week as the No. 1 player in the world on Monday.

The Serb is already by far the longest-reigning No. 1 in tennis history. His 420 weeks are 110 more than Federer had during his career. Pete Sampras is a distant third on the list with 286 weeks at No. 1.

Djokovic is just 12 tournament wins away from breaking Jimmy Connors' record of 109. He already holds the all-time record for most Grand Slams with 24, sitting two clear of Rafael Nadal. One more major would push him past Margaret Court for the most in the history of tennis.

With Federer retired and Nadal struggling with injuries, Djokovic is the last of the Big Three regularly on tour. He has not set a date on his own retirement but admitted it does weigh on his mind.

"To be honest, I'm kind of divided [on retirement]," Djokovic said in January. "There's always a part of me that is a young boy who just adores tennis and knows only about tennis and devoted his entire life to tennis and that boy still wants to keep going.

"And then, on the other side, I'm the father of two children, and I'm away from my family, and every time I travel for an extended period it breaks my heart. So I'm always thinking about, How long should I play, how many tournaments should I play, is it worth it?"

Perhaps the last box to check for Djokovic will be becoming the oldest player in history to win a Grand Slam. Ken Rosewall currently holds that record at 37 years, one month and 24 days.

With Djokovic's 37th birthday coming up next month, we might get to see that record fall sooner rather than later.