Alexander Zverev is guaranteed to finish the 2026 season as world number one after Carlos Alcaraz's surprise defeat in the Shanghai Masters.
German Zverev was on course to climb above current world number one Jannik Sinner, who has been ruled out for the rest of the year with a knee injury.
Alcaraz still had a mathematical chance of stopping Zverev until he was beaten 7-5 7-6 (7-4) by Belgium's Zizou Bergs in the third round on Monday.
"It's been a fantastic season for me. Knowing that I will finish the year on top of the rankings is just a dream come true," said Zverev.
After losing three major finals, Zverev landed the first Grand Slam title of his career by winning the French Open in June, and followed that with victory at the US Open last month.
The 29-year-old has taken advantage of the injury problems suffered by Spaniard Alcaraz and Italian Sinner - the dominant pair on the ATP Tour in the previous two seasons - to become the world's leading player.
Zverev cannot overtake Sinner until 2 November at the earliest.
He will become the first German man to end the year as number one, and the oldest player to achieve the feat for the first time.
After suffering a serious ankle injury in the 2022 French Open semi-finals which kept him out for seven months, Zverev regained his place in the upper echelons of the sport and been ranked inside the top four continuously since March 2024.
"So much work and effort have been put into achieving this goal, especially after coming back from the ankle injury. I look forward to finishing the season in style," he added.
At the start of the year it seemed unthinkable that anybody other than Sinner or Alcaraz would finish it as world number one.
Going into 2026, Alcaraz led the rankings with 12,050 points, closely followed by Sinner on 11,500.
The dominance of the pair - who split all of the eight Grand Slam titles in 2024 and 2025 between them - was illustrated by the huge gap to the rest of the field.
Third-ranked Zverev had 5,110 points - closer numerically to the 2,221st player in the rankings than the top two.
But as the season enters its final stretch, Zverev has completed what many thought was impossible.
There is no escaping the fact that Zverev has benefitted from Sinner and Alcaraz missing large chunks of the season through injury, meaning he won the French Open and US Open titles with at least one of the pair absent.
Both his major wins came without having to face Sinner or Alcaraz, and he did not face a top-10 player until beating Ben Shelton in the New York final.
Zverev has said he would "maybe not" be the world's best player if both his rivals were "healthy and at the top of their game".
But, as the old adage goes, you can only beat what is in front of you.
Zverev has done exactly that to record the greatest season of a career which had threatened to fall short of the lofty expectations placed on him since he emerged as a teenage prodigy.