Novak Djokovic certainly didn't waste time when he sailed into the professional waters, winning his first Futures title at the age of 16, the first Challenger just a day after turning 17 and the first ATP title back in Amersfoort in 2006. His debut on the ATP rankings list came in July 2003 and just two years later he was already ranked inside the Top 100, making a rapid expansion towards the best players on the Tour before 2007 when he established himself as the third strongest power on the Tour behind Roger Federer and Rafael Nadal.
Novak won almost 70 matches that year and he opened it with a title in Adelaide that pushed him inside the Top 15 for the first time in his career. That was on January 8 and Novak would spend almost 600 weeks in that company, or 11 years and 4 months! The years have gone by (eight from those 15 players are retired) and Novak has kept his place in the Top 15 without any problems, together with Rafael Nadal, but that had come to its end eventually, and that day will be the next Monday.
Namely, when the new ATP rankings list comes out on May 14, we would not find Novak's name among the Top 15 players after he suffered another early exit in Madrid, losing to Kyle Edmund in the second round to stay on just six wins from 12 matches in 2018. As we all know, Novak has decided to skip the second part of 2017 due to an elbow injury and Madrid is only his 10th tournament in the last 12 months, playing 31 matches in that period and winning 22 matches, which isn't enough to keep him in the Top 15 anymore.
In addition, Novak should lose his place in the Top 20 as well (has been ranked inside the Top 20 since October 2006) as he has to defend 600 points from Rome where he played in the final 12 months ago. Djokovic has spent 223 weeks at the top of the ATP rankings, finishing four times as the year-end number 1 and staying in the Top 10 for more than 10 and a half years (March 2007 - November 2017) but he is yet to find the right form since he made a comeback and it would be interesting to follow his progress through the rankings if he comes at least close to the level from 2015 and early 2016 when he ruled the tennis world like almost no one before him.