Andy Murray beat Novak Djokovic in straight sets in the World Tour Final - sealing his position as world number one for the rest of 2016.
The Wimbledon champion, who shot to the top of the world rankings a fortnight ago, would have been knocked off his spot if Djokovic had won.
It is the fifth title in a row the Scotsman has won, bringing his winning run to 24 matches.
Speaking at the O2 Arena in London, Murray, 29, said: "Obviously it's a very special day.
"Playing against Novak in a match like this, we've played grand slam finals, Olympics - it's been a tough rivalry.
"I've lost many of them - I'm very happy I managed to win today.
"To finish the year number one is something I never expected."
Andy Murray fought with gritted teeth and weary limbs to hold onto his number one dreams and reach his first final at the ATP World Tour Finals.
Four days after he took three hours and 20 minutes to defeat Kei Nishikori, Murray was on court for 18 minutes longer in a 5-7 7-6 (7/5) 7-6 (11/9) victory over fourth seed Milos Raonic - breaking the tournament record for longest match for the second time.
Raonic twice broke Murray when he served for the match and saved three match points but could not take the one chance that came his way in a remarkable deciding tie-break that eventually went the way of the home favourite.
Murray cut a fatigued figure throughout and looked exhausted by the end, and the question must now be how much energy he will have left for Sunday's final.
Jamie Murray and his Brazilian partner Bruno Soares will finish the year as the world's top-ranked doubles pair.
The duo went into the ATP World Tour Finals trailing Nicolas Mahut and Pierre-Hugues Herbert by 575 points but overhauled them on Thursday courtesy of their third win.
The French duo were eliminated after losing both their first two matches but could still have prolonged the race by winning their final group match against Henri Kontinen and John Peers.
But they lost 6-7 (5/7) 6-4 10-4, meaning Murray and Soares cannot be overtaken.
Murray became the first British player ever to be officially ranked world number one when he moved to the top of the individual doubles rankings earlier this year, pipping brother Andy to the honour.
Soares could now finish the year with that title if he and Murray win the title on Sunday.
Mahut only needed one victory to become the first French player to finish a year ranked number one but the Wimbledon champions have been well below their best at the O2 Arena.
Murray and Soares will play either Feliciano Lopez and Marc Lopez or Raven Klaasen and Rajeev Ram in the semi-finals on Saturday.
Andy Murray defeated Stan Wawrinka to surge through to the semi-finals of the ATP World Tour Finals, closer to a showdown with Novak Djokovic.
After winning his opening two matches against Marin Cilic and Kei Nishikori, the Scot knew a set against Wawrinka would be enough.
He went one better, clinching a 6-4 6-2 victory to set up a rematch of the Wimbledon final against Milos Raonic on Saturday.
Should Murray come through that, and Djokovic defeat Nishikori in the other semi-final, then Sunday's final would also be a straight shoot-out for world number one.
It was also a 22nd consecutive victory for Murray, equalling his career-best streak from this summer, which included titles at Queen's and Wimbledon and Olympic gold.
The Scot said: "The first set was tight, there weren't many chances either way. I just managed to get the break. In the second set, I'd already qualified, Stan obviously needed to win so it was a bit easier for me to play more freely than him.
"I played a very solid match and obviously I'm very happy to be through to the semis."
Judy Murray has hailed her son's "incredible" year after the double-Olympic champion was crowned world number one for the first time.Read the full story ›
British tennis star Andy Murray has beaten John Isner to win the Paris Masters.Read the full story ›
Andy Murray is one win away from topping the men's tennis world rankings for the first time.
Novak Djokovic's defeat at the Paris Masters means the Serbian's two-year reign as the world number one could end if Murray claims victory in the event's semi-finals on Saturday.
Three-time grand slam winner Murray came through his quarter-final with Tomas Berdych 7-6 7-5 while 12-time slam winner Djokovic lost 6-4 7-6 to Marin Cilic.
"Maybe a little bit before the match there were a few more nerves but once I got out there I didn't let it affect me," the Scot said.
"My goal wasn't to get to number one this year, I was looking at the beginning of next year as the chance to do it. If it happens this week then great."
Murray will play Jo-Wilfried Tsonga or Milos Raonic in Saturday's semi-final, which begins at 3.30pm, where victory would give him enough ranking points to surpass Djokovic.
Andy Murray survived a stern examination of his world number one credentials from Fernando Verdasco to reach the third round of the BNP Paribas Masters.
Murray needs to at least reach the final in Paris to stand a chance of overhauling Novak Djokovic on Monday and becoming the first British player ever to be ranked number one in singles.
That looked highly unlikely when a weary Murray faced two break points at 5-5 in the deciding set against an inspired opponent, but the Scot held on and ground out a 6-3 6-7 (5/7) 7-5 victory.
It was a 16th win in a row for the 29-year-old, who is bidding for a fourth straight title after winning trophies in Beijing, Shanghai and Vienna last week.
Andy Murray took a step closer to overtaking Novak Djokovic as world number one by winning his third successive title at the Erste Bank Open in Vienna.
The Scot defeated Frenchman Jo-Wilfried Tsonga 6-3 7-6 (8/6) in the final to add to the titles he won in Beijing and Shanghai earlier in October.
Murray will top the rankings for the first time in his career if he wins next week's Masters event in Paris, and Djokovic fails to reach the final.
It has been a remarkable last six months for Murray, who has now won seven titles, the most in a season in his career, and lost just three matches since the French Open in June.
Andy Murray closed in on world number one Novak Djokovic after winning his sixth title of the year with victory at the Shanghai Masters.
The Scot beat Roberto Bautista Agut, Djokovic's semi-final conqueror, 7-6 (7/1) 6-1 in 96 minutes to extend his winning run to 10 matches after triumphing at the China Open in Beijing a week ago.
Victory in Shanghai, Murray's 41st career title, cut the gap to Djokovic to 915 points.
The first set went with serve until Murray broke his Spanish opponent to take a 4-3 lead but when Bautista Agut broke back and then made it 6-5, Murray took the first set to a tie-break with a love game.
He cruised through the tie-break 7-1 to make it 19 sets won in a row before three breaks of service, to his opponent's one, in the second set eased the Briton into a 5-1 lead from where he closed out the match with a number of 100mph second serves and powerful groundstrokes.