Rafael Nadal admits he "never dreamed" of ending 2017 as the top-ranked player in men's tennis.
Nadal ensured he would go into 2018 at the rankings summit by beating Chung Hyeon in straight sets in the second round of the Paris Masters on Wednesday.
The victory makes the 16-time grand slam champion the oldest man to achieve the feat and the seventh to do so on four or more occasions, having previously come out on top in 2008, 2010 and 2013.
Nadal started the year down in ninth after prematurely ending his 2016 campaign to allow a wrist injury to recover and concedes he could not have foreseen such a sharp change in fortunes 12 months ago.
"I'm very, very happy for everything," he said on court following the victory over Chung. "It has been an amazing year.
"One year ago, for sure, I never dreamed about being world number one again at the end of the season.
"It's something that means a lot to me. But the season is not over."
Nadal replaced Andy Murray at the top in August and has won six titles this year, including two grand slams and two Masters 1000 events.