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.
On Monday he will be officially confirmed as Britain's first singles number one since computerised rankings were introduced in 1973.
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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.
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.
The draw for that competition takes place on Monday at 15:00 GMT.
Murray made to work for prize
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.
The unseeded Isner continued to put pressure on Murray in the second, showing an increasing deftness at the net to take the second set to a tie-break.
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.
What they said
Murray: "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: "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."
The stats behind Murray's rise to number one
- He is the 26th man to hold the top spot since computerised rankings began in 1973.
- A player's ranking is determined by his best 18 tournament results over the preceding 52 weeks.
- 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 holds the record for the longest time between first becoming number two and becoming number one - seven years and over two months.
- His seven stints at number two are tied with Mats Wilander and Stefan Edberg for the longest in the ATP's database of week-by-week records, which goes back to June 1984.
- Seven players since June 1984 have never become number one after becoming number two: Michael Stich, Goran Ivanisevic, Michael Chang, Petr Korda, Alex Corretja, Magnus Norman and Tommy Haas.
Simon Briggs, Daily Telegraph tennis correspondent on BBC Radio 5 live sports extra
It 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 today.
There has not been too much to warrant criticism for him this season. He lost in the second round of Miami and Indian Wells following the birth of baby Sophia. He said it gave him a better perspective on the world and this is a baby bounce.