His late-season struggles help explain why he has never lifted the title.
Nadal has a 51-6 record and has won four titles this season, including a record-extending 12th French Open and a fourth US Open. He finished as the runner-up in 2010 and 2012. The idea is to be able to play the ATP Finals in London.
The pair will do battle in the Bjorn Borg group at the O2 Arena and are joined by Dominic Thiem and Italian debutant Matteo Berrettini.
In what feels like a familiar November story, Rafael Nadal is travelling to London on Wednesday in the hope of playing in next week's Nitto ATP Finals, despite an abdominal strain. He has previously made two finals, the last one in 2013.
Nadal pulled out of last week's Paris Masters due to injury but revealed on Twitter that he would come to London this week.
If fit, Nadal will face final year's champion - 22-year-outmoded Alexander Zverev - in the final of the hole sequence of round robin fits, to be performed on Monday night time.
"I think the Andre Agassi group in very interesting with three young guys and Rafa in there".
If Nadal fails to win even one round robin match, Djokovic could be crowned world number one simply by reaching the final after two group-stage wins. The ATP finals take place between November 10-17. Although the 32-year-old found himself pushed into second spot by Nadal after the Paris Masters, victory in London would nearly certainly help him climb to the top of the pile.
While the Greek may not be as settled or consistent as some of his peers, like Medvedev and Thiem, he definitely has proven himself capable of defeating any and all of the contenders in London. With a lot of unpredictability and uncertainty to look forward to, a tennis fan will be delighted as that is precisely what would make for a phenomenal end to another spectacular year on the ATP calendar.