London - Novak Djokovic revelled in his "perfect" five months on Friday as he took aim at his sixth ATP Finals title with the year-end No 1 ranking in the bag.
The Serb, who won Wimbledon and the US Open, earlier this week replaced the injured Rafael Nadal at the top of the men's rankings and is guaranteed to end 2018 in top spot.
Speaking before the start of the season finale at London's O2 Arena, the Serb said next to the Grand Slams and the ATP Finals, reaching the number one spot was the "ultimate challenge" in the sport.
"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.
"After February's elbow surgery it looked quite improbable that I would be in this position as the year-end number one, not just because of the rankings - I was 22 (in May) - but just because of how I felt on the court, how I was playing.
"But there was always part of me that believed I could make it back and I never thought it was impossible. I just thought at the time it might take more time than I wished it to be.
"It turned to out to be a perfect five months of the year, with two Grand Slam titles," he added.
"Of course, unfortunately Nadal had to withdraw from all of his tournaments after the US Open so he was not in that race but at the same time I thought it was a great five months that is crowned with this achievement."
The 31-year-old Djokovic underwent surgery on his right elbow in January after the Australian Open, which was his first tournament in six months.
He struggled to find his best tennis when he returned to the court but reached the final of Queen's before winning Wimbledon and the US Open.
Since the start of Wimbledon he has won 31 out of 33 matches.
Djokovic is seeking his fifth ATP Finals title in London to go with his first, which he won in Shanghai in 2008.
If he wins the tournament, which starts on Sunday, he would draw level with Roger Federer with six wins in total.
Djokovic is in Group Guga Kuerten along with Alexander Zverev, Marin Cilic and John Isner while Federer is in Group Lleyton Hewitt with Kevin Anderson, Dominic Thiem and Kei Nishikori.
Each player plays the other group members, with the top two advancing to the semi-finals.
Djokovic also confirmed that he would not be playing Nadal in a controversial exhibition match in Saudi Arabia in December because of the Spaniard's injury.