With Djokovic losing to Tsitsipas at the Shanghai Masters QF, Nadal's lead over Djokovic is 1,280 points going to the latter stages of the 2019 season. Technically, Djokovic can still finish 2019 as YE#1 however, he literally has to win BOTH Paris-Bercy AND World Tour Finals (he has to go undefeated here). If Djokovic can not find the pre 2016 level, then Nadal will take home the YE#1 ranking for 5th time of his career.
As for Nadal, he is confirmed to play both Paris-Bercy and World Tour Finals so he will only add to his lead. I would say he will play a couple of matches in Paris and then he will really focus on World Tour Finals.