Rafael Nadal became the oldest year-end world number one since rankings began by beating Chung Hyeon 7-5 6-3 in the second round of the Paris Masters.
The withdrawal of Roger Federer from the tournament meant victory for Nadal in this match was enough for the Spaniard to end the season at the top of the rankings for the fourth time in his career.
Defeat would not have spelt the end of the 31-year-old's chances of achieving the feat, and he was put through his paces early on by his South Korean opponent.
It was Chung, 21, who broke first, going 2-1 up in the opening set before conceding four successive games to the 16-time grand slam champion.
Chung hit back with a stunning break to love to get back on terms with Nadal, who then survived a break point at 5-5 as the set appeared destined for a tie-break.
However, Nadal gathered his thoughts at the change of ends and made light work of the 12th game - wrapping up the set with a win to love on Chung's serve.
Nadal faced no break points in the second set but squandered four chances to edge 3-1 in front as Chung continued to put in a creditable performance against a man he had faced just once before, losing a quarter-final in Barcelona earlier this year.
But he made no mistake when the opportunity next fell his way, opening up a 5-3 advantage before sealing the win - and the number-one ranking - with his second match point.