The 32 year-old Spaniard retired on Friday night when tailing the towering Argentinian 6-7, 2-6, with a recurrence of knee trouble that made it impossible to continue.
Djokovic, who beat Nishikori in the Wimbledon quarters, said he expected a tough match against an opponent with a lethal two-handed backhand who is one of the fastest players on tour.
At the time of writing, Nadal is the [1.70] favourite and I make this some value. I had an issue in the second or third match. In the second or third match, then I think it was a little bit better.
"I had my biggest memories on this court in 2009 when I beat Rafa and Roger (Federer, in the final)".
'I said to my box immediately that I felt something on the knee.More news: Elon Musk smokes weed in California-based podcast appearance
"I waited as much as I can, very hard to me to say goodbye before the match finished". But it was not the day.
"It was hard for me to keep playing". But at some point, you have to take a position. "I know I'll have to play at my highest level to keep having chances of success".
On his first service game after the medical intervention, Del Potro struck.
"But my personal feeling is I going to be at my 100 per cent for the next one". I know how much frustration there can be when you can't do the thing that you want to do. For him will be huge if he's able to win again a Grand Slam'. I still here. I still here because I love what I am doing'.
Nadal would figure to have a tougher time in his fourth meeting with del Potro in the last five Grand Slam tournaments.More news: Delta Fire Maps: Location, Evacuations, & Updates
On Sunday, the powerful Argentine with the massive forehand that zips along at more than 100 miles per hour (160 kph) will face two-time U.S. Open champion Novak Djokovic or 2014 runner-up Kei Nishikori.
It was a high-quality set of tennis, with more winners than unforced errors on each side and with a lot of great points and attractive winners.
The Spaniard had tape applied here after seven games only to take it off two games later.
And after an exchange of breaks in the first two games, the opener became perfectly poised until Del Potro unleashed his formidable forehand to move ahead in the ninth game, but he then failed to take two set points when serving for the set. The anguished looks to his support box spoke of his unease.
In these, Nadal eliminated Del Potro at this stage last year here - priced much shorter at [1.36], before eliminating him in this year's French Open and Wimbledon - again priced considerably shorter than market lines. I am sad for him. The key to the match was the first set. I did well in the tiebreak and I am so happy to be in the final once again. I didn't expect to get in another grand slam final.More news: Cher coming to St. Louis in 2019