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 the last couple of years.
"But yes, here we are, (it) happened. And very happy of course. Means a lot, no? After almost 10 years since the first time that I finished the year with this number."
Nadal went into the match knowing victory would give him an unassailable lead as world number one over old rival Roger Federer, after the Swiss legend withdrew before the start of the Paris tournament.
The 21-year-old Chung put up a brave fight, but a fired-up Nadal proved too hot to handle.
Nadal will next face Pablo Cuevas in the last 16 on Thursday, after the unseeded Uruguayan edged out Albert Ramos-Vinolas 6-7 (5/7), 7-6 (7/1), 6-2.
Victory in Sunday's final in Paris would see Nadal move clear of Novak Djokovic with a record 31st Masters crown.
"I am on a good dynamic, playing well the whole season," Nadal, who has only won two of his 75 ATP titles indoors, added.