Novak Djokovic celebrated a special hat-trick after ensuring he will finish a year as world number one for the third time with victory over Tomas Berdych in London.
Djokovic's 6-2 6-2 win also booked his place in the semi-finals of the ATP World Tour Finals but that was all but guaranteed anyway and it was the ranking milestone that was on his mind as he joined his coaching team for a celebratory huddle.
The Serbian was joined on court at the O2 Arena by his head coach, Boris Becker, and three other former number ones, John McEnroe, Mats Wilander and Carlos Moya, for the presentation of a trophy marking his achievement.
Djokovic topped the rankings in 2011 and 2012 before narrowly missing out to Rafael Nadal 12 months ago.
It has been a landmark year on and off the court for Djokovic, who won a second Wimbledon title only a few days before marrying his long-time partner Jelena.
The couple celebrated the birth of their first child, Stefan, three weeks ago.
Djokovic said: "It's a great deal of happiness knowing what I've been through the entire season in order to get to where I am and finish the year as number one. I owe gratitude to my team, to all the people who were next to me.
"Being number one of the world is the pinnacle of the sport. Winning grand slams also is something that allows you to write the history of this sport.
"It's difficult to say what is more important. But both these goals are always my goals at the beginning of the season. So I'm glad I managed to achieve both."
Djokovic made the surprise appointment of Becker late last year and their partnership took a while to gel. The 27-year-old then lost a grand slam final for the fifth time in six attempts to Nadal at the French Open. But a month later he edged a brilliant five-set encounter against Roger Federer to win Wimbledon and victory at the Paris Masters a fortnight ago, plus his three wins here, were enough to ensure he stayed ahead of the Swiss in the rankings.
Djokovic said: "I think I had a great season and great year overall for my private life and professional as well. Considering all the things that have happened and circumstances, I thought I have done very well."
Djokovic now meets Kei Nishikori in a rematch of the US Open semi-finals, where the Japanese player caused a stunning upset.
In the other semi, Roger Federer will face Stan Wawrinka after his fellow Swiss won the three games necessary to qualify for the last four in his final match against Marin Cilic.