"I'm here to try my best - as in every tournament", he said before starting as top seed in the event which is missing Roger Federer due to fatigue.
"It was impossible for me to think about (being No. 1) coming back from a tough period without playing, and with so many injuries in the last couple of years", said Nadal, who also finished as No. 1 in '08 and '10.
The 31-year-old Nadal will be the oldest year-end number one and first season-ending number one over the age of 30 since the Emirates ATP Rankings were established in 1973.
Nadal was also crowned year-end No.1 in 2008, 2010, 2013, his title this year is incredibly impressive considering he was seeded No.9 at the Australian Open earlier this year.
"It has been an awesome year".
With his victory of the day Rafael Nadal secures his place as number 1 in the world in 2017... and this for the 4th time!
"One year ago, for sure, I never dreamt about being No. 1 again at the end of the season so it's something that means a lot to me".
The world No. 1 has six ATP singles titles on the year including two Grand Slams.More news: Preterm birth rate up across Virginia
The 21-year-old Chung put up a fearless fight, but a fired-up Nadal proved too hot to handle.
Nadal was broken when serving for the set as Chung hit back, but the turning point came as the world number one saved a break point in game 11, marked with a trademark fist-pump.
After Chung fended off four break points in the fourth game of the second set, Nadal made his move with a break in the ninth game.
Nadal has enjoyed fantastic success in Paris, winning the French Open 10 times at Roland Garros, but he has never won the title at Bercy. The Majorquain will play Pablo Cuevas in the knockout stages.
Argentinian del Potro, who meets Joao Sousa this morning, needs to reach his fifth successive semi-final to overhaul Carreno Busta.
Isner, who lost last year's final to Murray, next plays either No. 6 Grigor Dimitrov or Richard Gasquet.
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.