The ATP said it believes it's the first time the men holding the top two spots in the singles rankings have partnered for doubles at a tournament since No. 1 Jimmy Connors and No. 2 Arthur Ashe did it in 1976.
Benito Perez-Barbadillo, the publicist for Nadal and Djokovic, said the pair discussed the possibility of playing with each other last season at hard-court events in Montreal or Cincinnati. But Nadal wound up having a stomach muscle problem, so those plans were scuttled.
Djokovic brought up the thought of a high-powered doubles team again recently.
"Novak called Rafa a couple of weeks ago and reminded him," Perez-Barbadillo wrote in an e-mail to the Associated Press on Saturday. "Rafa liked the idea again and here they are."
Nadal returned to No. 1 in the ATP singles rankings in June, replacing Roger Federer there, after winning his fifth French Open championship. When Federer then lost in the quarterfinals at Wimbledon, that made him slide to No. 3 for the first time since 2003, and Djokovic moved up to No. 2.
Nadal owns eight Grand Slam singles titles, and Djokovic has one.
Neither has won a major championship in doubles; generally, the best male singles players do not want to expend extra energy by entering the doubles competition at Grand Slam tournaments.
Nadal does own six career ATP doubles titles, including one in 2010, at Indian Wells, Calif., in March, when he played with Marc Lopez. Djokovic's lone ATP doubles title came this year at Queen's Club in June, alongside Jonathan Erlich.
The Toronto tournament starts Monday.
Connors and Ashe teamed up at a tournament in North Conway, N.H., in August 1976, losing in the quarterfinals. In July 1976, they lost in the final at Washington.