Rafael Nadal secured the year-end world number one ranking with a 7-5, 6-3 second-round victory over South Korean Hyeon Chung at the Paris Masters on Wednesday.
The 16-time Grand Slam champion is the oldest man to finish a season at the top of the rankings at the age of 31, after a year that saw him win the French and US Opens.
It is the fourth time the Spaniard, who started the season ninth in the rankings, has achieved the feat after 2008, 2010 and 2013.
“Of course it was not the goal (at the start of the season),” Nadal told a press conference.
“It was impossible for me to think about that when you are coming back from a tough period of time without playing tennis and with so many injuries in last couple of years.