Rafael Nadal is guaranteed to finish the year as world number one after beating Hyeon Chung to reach the third round of the Paris Masters today.
The Spaniard will duly became the oldest year-end world number one and ensured it will be the first time a 30-something has achieved the feat since the tennis rankings were established in 1973.
He was pushed hard in the first set before easing through the second to win 7-5, 6-3 against his South Korean opponent at the AccorHotels Arena in this Association of Tennis Professionals World Tour Masters 1000 event.
It also marks the fourth time Nadal has ended the year as world number one after also doing so in 2008, 2010 and 2013.
It follows the withdrawal of Roger Federer before the start of the Paris event.
"I'm very, very happy for everything," said Nadal afterwards.
"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."
Dutchman Robin Haase ousted German fourth seed Alexander Zverev 3-6, 6-2, 6-2 elsewhere today.
American John Isner also hit 29 aces in beating Argentinian Diego Schwarzman 7-6, 6-7, 6-3 in an epic duel lasting 2 hours 35min.
Three matches also took place today as competition continued at the Women's Tennis Association Elite Trophy at the Hengqin Tennis Centre in Zhuhai, China.
Home US Open champion Sloane Stephens was easily beaten 7-5, 6-3 by Latvia's Anastasija Sevastova.
French top seed Kristina Mladenovic was also defeated by Slovakia's Magdalena Rybarikova, 7-5, 1-6, 7-6.
Australia's rising star Ashleigh Barty cruised to victory over Russian Anastasia Pavlyuchenkova, 6-4, 6-1.
Action will continue tomorrow in both events.