Rafael Nadal's return to the world number one position for the eighth time in his career was confirmed when the ATP rankings were released on Monday.
The Spaniard leapfrogs Novak Djokovic even though the Serb sealed his fifth Paris Masters title on Sunday with a 6-3, 6-4 victory over Canadian Denis Shapovalov.
Nadal, whose last stint at the top ended a year ago on November 4, 2018, withdrew from his semifinal against Shapovalov in Paris with an abdominal muscle strain.
Djokovic slips down the rankings because he loses the points that he won a year ago in London when Nadal was absent.
The 2019 battle between Nadal, winner of the French and US Opens, and Djokovic who triumphed in Australia and Wimbledon, is set to go to the wire as either could still finish the year at number one.
If the Spaniard does not play or fails to win a round-robin match at the World Tour Finals, Djokovic must claim two group-stage victories and make the final to pass him.
Djokovic would also be sure of the year-end top ranking should he win the tournament and Nadal does not reach the semis.
Nadal first reached number one on August 18, 2008 when he was just 22. He has spent 197 weeks at the top, the sixth highest in ATP history behind Roger Federer (310), Pete Sampras (286), Djokovic (275), Ivan Lendl (270) and Jimmy Connors (268).
Gael Monfils returns to the top 10 for the first time since February 2017 after reaching the semifinals in Paris while 20-year-old Shapovalov climbs 13 places to a career-high 15.