Alexander Zverev beat Tallon Griekspoor 6-4, 7-5 on Thursday to reach the Vienna Open quarter-finals and secure the ATP world number one ranking for the first time.
The 29-year-old German claimed titles at the French and US Opens earlier in the year and will become the 30th man to hold the top ranking since 1973.
Zverev will be aged 29 years and 196 days when the new rankings are published, making him the second-oldest first-time number one after John Newcombe.