That is because Rafael Nadal has withdrawn from the Paris Masters.
Djokovic is currently second, but will leapfrog the Spaniard whatever happens in this week’s tournament.
Nadal has now withdrawn or retired from four out of five hardcourt tournaments.
The only one he played in, the Canadian Open, he won by defeating Stefanos Tsitsipas in August.
Djokovic has made history with his climb to the summit of tennis.
Earlier in the season, the Serb was ranked 22nd, the lowest anyone has ever come from to be number one in the same season.
More follows.