Britain’s Andy Murray celebrated his rise to world number one by beating American John Isner 6-3 6-7 (4-7) 6-4 to win his first Paris Masters title.
The victory brought Murray his sixth ATP title of a memorable 2016.
The Scot, 29, also added his second Olympic and Wimbledon titles on his way to the top of the rankings.
Murray’s ascent was confirmed by Milos Raonic’s withdrawal from Saturday’s semi-final with a leg injury, but the hype around his accomplishment did nothing to knock his concentration.
Watch Andy Murrays amazing journey to world no.1
He now heads to the season-ending World Tour Finals as top seed for the first time – but if previous number one Novak Djokovic wins every match at the Finals he will reclaim the top ranking he held for 122 weeks.
After double-faulting on the first point of the first game, Murray soon clicked into gear, breaking for a 4-2 lead, and showing watertight defence to close the door on Isner when the American created two break points of his own in the next game.
Murray managed only six points on his opponent’s serve in the set, but that was enough to take it in 35 minutes.
And the 6ft 8in 31-year-old had too much for the Scot in the decider, serving imperiously to take the breaker 7-4.
Twice in Isner’s first two service games of the third set Murray saw break points snatched away by his opponent’s huge serve.
But Murray was not be denied, and he finally forced a break at 5-4, firing in a rapid backhand that Isner could only dig into the net, to take the third set and the match.
Isner was looking for his first title in a season in which his Paris semi-final against Marin Cilic was his only victory against a top-10 opponent
Murray in a statement said, “To my team and my family, this has been an incredible journey to get to the top of the rankings. I could not have done it without you. They make a lot of sacrifices to allow me to compete and travel the world. I will work as hard as I can to continue getting better.”
Isner also in a statement said, “Well done to Andy Murray for the title and getting to number one in the world. What an incredible achievement.
“Every single week I am in the same locker room as you, and see how how hard you work, you deserve it.”
Murray’s is the 26th man to hold the top spot since computerised rankings began in 1973.
Murray is the second-oldest player to debut at number one behind John Newcombe, who was 30 years and 11 days old when he achieved the feat in 1974.
Murray has now won seven of the nine ATP World Tour Masters titles, with Monte Carlo and Indian Wells missing from his trophy cabinet
Murray is extraordinary the way the whole season has switched. Novak Djokovic was in the ascendancy, but Andy Murray is number one in every way.
He is dominant in the play, in aura, in the locker room and on the points table. No-one wants to play him; they would rather play Novak and it has been shown why in this Paris masters finals.