I just thought at the time it might take more time than I wished it to be.
Djokovic looks the man most likely to deny Federer a 100th Tour title, having resumed the dominance he displayed in 2011, 2015 and 2016.
Meanwhile, Djokovic, who has made an incredible comeback this year after an elbow injury, leads head-to-head against Federer 25-22, the last win coming at the semi-final of the Paris Masters last week.
The ATP Tour Finals cap off a strong season for the big-hitting American who achieved a career high of eight in the rankings, and defeated Cilic, del Potro and Zverev on his way to winning his first Masters 1000 title in Miami in April. He has won four this year, including the Australian Open, in what has been a less consistently transcendent campaign.
'But it's been great here, I've enjoyed playing here.
"If they stay I think it's definitely a good choice", said the Swiss.
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.
His first opponent, Kevin Anderson, has qualified for just the first time in his successful career.
"It turned to out to be a ideal five months of the year, with two Grand Slam titles", he added.
"It seemed to me that Sampras and Agassi had to be out of the picture before Federer gained the confidence to win".
"It's really only Novak (Djokovic) who can say at this point "If I play as well as I have done the last few months maybe I can also win the World Tour Finals" but not even that is a given because, especially in the best-of-three-set matches indoors, margins are slimmer so you can slip fast". This is the third year running that eight different countries have been represented in the singles field.
The 37-year-old takes on Kei Nishikori for the tenth time.
The ATP Finals, as it is now known, began in Tokyo in 1970 and spent six years on the road before a 13-year stint in NY and a six-year stay in Frankfurt before spells in Hanover, Lisbon, Sydney, Houston and Shanghai. With Rafael Nadal out of the mix this year, Djokovic and Federer are the players to beat and Thiem could be just the player to make an impact. After six hours and 36 minutes, the South African prevailed 7-6 (6), 6-7 (5) 6-7 (9), 6-4, 26-24.