Paris - 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.
Former US Open champion Juan Martin del Potro gave his World Tour Finals bid a boost with a 6-2, 6-2 win over Portuguese qualifier Joao Sousa.
The 13th-seeded Argentinian needs to reach at least the semi-finals to take the eighth qualifying spot from Pablo Carreno Busta, who was dumped out by French wildcard Nicolas Mahut on Tuesday.
The 29-year-old was 47th in the points standings before the US Open in August, but an excellent run of form during which he reached the semi-finals at Flushing Meadows and won the Stockholm Open title has dragged him back into contention.
Del Potro will face unseeded Dutchman Robin Haase in the last 16.
But Frenchman Jo-Wilfried Tsonga failed in his efforts to reach the season-ending eight-man event with a surprise 2-6, 7-6 (7/4), 6-2 defeat by compatriot and world number 83 Julien Benneteau.
Another home player, Lucas Pouille, kept his slim hopes of reaching London alive by seeing off Spaniard Feliciano Lopez 6-3, 6-4, but US Open runner-up Kevin Anderson's chances ended with a 5-7, 6-4, 7-5 loss to Fernando Verdasco.
Bulgarian world number eight Grigor Dimitrov, who has already booked his ticket for the Tour Finals, knocked out French favourite Richard Gasquet with a straightforward 6-4, 6-4 win.
Earlier on Wednesday, fourth seed Alexander Zverev was dumped out by Haase in a 3-6, 6-2, 6-2 second-round defeat.
The 20-year-old German lost his composure after easing through the first set, as his bid for a third Masters title of the season was abruptly ended.
Results from the ATP Paris Masters on Wednesday (x denotes seeding):
Rafael Nadal (ESP x1) bt Hyeon Chung (KOR) 7-5, 6-3
Pablo Cuevas (URU) bt Albert Ramos (ESP x15) 6-7 (5/7), 7-6 (7/1), 6-2
Robin Haase (NED) bt Alexander Zverev (GER x4) 3-6, 6-2, 6-2
Juan Martín Del Potro (ARG x13) bt João Sousa (POR) 6-2, 6-2
John Isner (USA x9) bt Diego Schwartzman (ARG) 7-6 (7/2), 6-7 (11/13), 6-3
Grigor Dimitrov (BUL x6) bt Richard Gasquet (FRA) 6-4, 6-4
David Goffin (BEL x7) bt Adrian Mannarino (FRA) 6-2, 2-6, 6-3
Julien Benneteau (FRA) bt Jo-Wilfried Tsonga (FRA x11) 2-6, 7-6 (7/4), 6-2
Roberto Bautista Agut (ESP x14) bt Jérémy Chardy (FRA) 6-4, 6-1
Marin Cilic (CRO x3) bt Borna Coric (CRO) 6-4, 6-3
Fernando Verdasco (ESP) bt Kevin Anderson (RSA x12) 5-7, 6-4, 7-5
Jack Sock (USA x16) bt Kyle Edmund (GBR) 4-6, 7-6 (7/4), 7-6 (7/5)
Lucas Pouille (FRA x17) bt Feliciano Lopez (ESP) 6-3, 6-4