That's because the man he overtook, Rafael Nadal, is done for 2018.
He had surgery on his right ankle Monday.
The 22-year-old Khachanov defeated the 31-year-old Djokovic 7-5, 6-4, denying the Serbian superstar a chance to win his fourth tournament title in a row.
He added that he made a decision to have the ankle operation so he can start 2019 healthy.
Rafael Nadal has withdrawn from the season-ending ATP Finals due to a persistent abdominal injury and will take the time off to have ankle surgery.
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It's the news every Girl Power fan has been waiting years for - the Spice Girls are officially getting back together. The Spice Girls have FINALLY confirmed they are reuniting for a major stadium tour across the United Kingdom.
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The voters apparently listened , passing Amendment 4, and re-enfranchising some 1.4 million potential voters. Another gambling amendment Florida voters approved Tuesday was Amendment 3.
Karen Khachanov stuns Novak Djokovic to win Paris Masters
Cilic wrapped up the opener in the 10th game, ending Wimbledon and US Open champion Djokovic's 30-set win streak in the process. The Serb seemed agitated at times and twice turned to his box to remonstrate about an unspecified issue during the first set.
Novak Djokovic of Serbia stretches for the ball return it to Karen Khachanov of Russian Federation during their final match of the Paris Masters tennis tournament at the Bercy Arena in Paris, France, Sunday, Nov. 4, 2018.
Nadal has played just nine events in 2018, winning five of them, including an historic 11th French Open title at Roland Garros for his 17th career grand slam.
Nadal - who has not featured on the ATP Tour since retiring ahead of his semi-final with Juan Martin del Potro at the US Open in September - posted an update to Twitter on Wednesday morning (AEDT), confirming he had returned following procedure.
The ATP Finals take place in London on November 11-18, with Group Hewitt matches kicking off the action on Sunday.
The Spaniard lost his place as world number one last week when injury forced him to pull out of the Paris Masters, with Novak Djokovic moving to the top of the rankings.
Novak Djokovic of Serbia returns the ball to Karen Khachanov of Russian Federation during their final match of the Paris Masters tennis tournament at the Bercy Arena in Paris, France, Sunday, Nov. 4, 2018.