Novak Djokovic says being back as world No 1 is "extra special" due to the fight he has faced to get back to the top of the game due to injury and loss of form.
The Serbian regained top spot in the ATP rankings earlier in November from Rafael Nadal with his run to the final of the Paris Masters, the first time he has been there for two years.
As he prepares to try and win the ATP Finals for a sixth time, which begin in London on Sunday, Djokovic, 31, was on reflective mood as he looked forward to the tournament.
"I'm very proud of that achievement and I understand that it's extra special this year because of the journey I've been through in the last 15 months, especially and particularly in the last eight to 10 months," Djokovic said.
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Ranked 22nd in June after two years of struggles with form and fitness that eventually saw him undergo elbow surgery, he has lost only two of his past 33 matches, winning back-to-back grand slam titles at Wimbledon and the US Open.
"It turned to out to be a perfect five months of the year, with two grand slam titles," Djokovic added.
The tournament will be Djokovic's last action of the season, with his next time on court being the Mubadala World Tennis Championship in Abu Dhabi between December 27-29.
A scheduled exhibition match with Nadal in Saudi Arabia on December 22 has been cancelled. The encounter in Jeddah has been prevented from happening due to Nadal having to end his season early due to an abdominal injury, as well as having ankle surgery.
Djokovic said: "Nadal is injured, so the match is not happening this year."
Djokovic is looking to join Roger Federer as the most successful player in the history of the tournament by winning his sixth title and fifth at the O2.
He lifted the trophy for four years in a row between 2012 and 2015, while Federer is bidding for his first title since 2011.
The pair are the top two seeds in the absence of the injured Rafael Nadal, with Djokovic in Group Guga Kuerten along with Alexander Zverev, Marin Cilic and John Isner, while Federer is in Group Lleyton Hewitt alongside Kevin Anderson, Dominic Thiem and Kei Nishikori.
Federer opens his campaign on Sunday evening against Nishikori, with Djokovic taking on Isner on Monday.
Should Federer, who lost narrowly to Djokovic in Paris last week, end his seven-year drought, he would become only the second man in the Open era after Jimmy Connors to win 100 singles titles.
It would be a fine way to end the season but the 37-year-old insisted he is not in a hurry to reach the milestone.
"I don't think it matters really where I win my 100th as long as it happens at one point," he said. "If I won here it's more about winning the World Tour Finals than my 100th."
