The Greek was unable to go all the way, losing to Rafael Nadal in the title match but achieving something that has not been seen that often at the events of the premium ATP series.
Rafael Nadal pulled out of the season-ending ATP Finals because of an abdominal injury and had surgery on his right ankle Monday.
Nadal issued a statement on social media this afternoon: "It has been a hard year - very good on a tennis level when I have been able to play and, at the same time, very bad as far as injuries are concerned".
The Spaniard's absence from London means 14-time Grand Slam champion Djokovic will end the year as the world number one for the fifth time in his career: level with Roger Federer and Jimmy Connors, and one behind Pete Sampras. "And, of course, I'm very, very happy and proud about it".
The tennis player explained the problem had been detected "a while ago and it occasionally bothered me".
When the 31-year-old from Serbia fell to No. 22 in May after beginning the year with a 6-6 record while recovering from surgery on his right elbow, it was his lowest ranking since he was 22nd in 2006 as a teenager. He hadn't been ranked that low since he was 22nd in 2006 as a teenager.
Any year in which you win an 11th title at Roland Garros, Barcelona and Monte Carlo is hardly a wasted one, and yet this has been a deeply frustrating year for Nadal.
Marat Safin went from No. 38 to No. 1 in 2000.
Last week, he had to forfeit the tournament of Paris-Bercy due to injury to the abdominal muscles.
He played his last match at the US Open in September, when he retired with a knee injury in the semifinal against Argentina's Juan Martin Del Potro.
Novak Djokovic of Serbia shouts during the final match of the Paris Masters tennis tournament against Karen Khachanov of Russian Federation at the Bercy Arena in Paris, France, Sunday, Nov. 4, 2018.