Andy Murray will become the first British singles player to be world number one since computerised rankings began in 1973.
The Scot, 29, needed to reach the final of the Paris Masters to replace Novak Djokovic – and did so when Milos Raonic pulled out before their semi-final.
Murray has reached 11 finals in his past 12 events, and has won 73 matches in the year, a personal record.
He will be officially confirmed as world number one on Monday.
Djokovic has topped the rankings for 122 weeks, and completed his career Grand Slam by beating Murray in the French Open final in June.
But Murray has since won his second Wimbledon and Olympic titles.
He will play John Isner in Sunday's Paris Masters final, after the American beat Marin Cilic 6-4 6-3 in the first semi-final.
Cilic had beaten Djokovic, 29, in the quarter-finals on Friday.
Raonic pulled out of Saturday's match with Murray after tearing a quad in his right leg.