This is the first time, though, that each of those three men spent some time at No. 1 during the same year. The seven changes there were the most since 1999, when there were eight.
Djokovic won Wimbledon and the US Open in 2018, while Federer took the Australian Open, and Nadal earned the French Open title.
They accounted for Nos. 1-3 in the final rankings from 2007 to '11 and 2014, too.
For Djokovic, it marks the fifth time he has closed a year in the top spot, following 2011, 2012, 2014 and 2015. Only Pete Sampras, with six year-end No. 1 finishes, did it more often.
The 31-year-old from Serbia is also the oldest man to be No. 1 at season's end and the first to finish atop the rankings after being outside the top 20 during the year -- or outside the top 10 at the end of the previous year (he was No. 12 in 2017).
Simona Halep ended the season at No. 1 in the WTA rankings for the second consecutive year; she clinched that spot more than a month ago. Angelique Kerber was No. 2 among women, followed by Caroline Wozniacki, Elina Svitolina and Naomi Osaka.
Djokovic overtook Nadal three weeks ago; the Spaniard did not compete anywhere after stopping because of a painful right knee during his U.S. Open semifinal in September against Juan Martin del Potro. Nadal had arthroscopic surgery on his right ankle this month.
Nadal concludes a season at No. 1 or No. 2 for the 10th time, trailing only Federer's 11.
Federer, meanwhile, is in the year-end top three for a record 14th time. The 37-year-old from Switzerland has won 20 Grand Slam singles championships, the most by a man, followed by Nadal with 17, then Djokovic and the retired Sampras with 14 apiece.
Del Potro was the runner-up at the US Open, Anderson at Wimbledon, Cilic at the Australian Open, and Thiem at the French Open.
Neither Anderson nor Isner ever had finished a year in the top 10, while del Potro is back in that group for the first time since 2013.