Novak Djokovic's withdrawal means Rafael Nadal will be the top seed in Montreal when main draw begins on August 5.
Rafael Nadal had played an outstanding level of tennis at the tournament without dropping a set until he met Novak Djokovic.
Rafael Nadal was forced to withdraw from his US Open semi-final against Juan Martin del Potro in early September with a knee injury.
Juan Martin Del Potro, the US Open runner-up to Novak Djokovic this year, hobbled out of the Shanghai Masters last 16 against Borna Coric on Thursday.
Argentine top seed Del Potro was far from his best and has been fighting flu or a cold for several days in the Chinese capital.
Del Potro, the 2009 US Open winner, was beaten by Djokovic in the final at Flushing Meadows in early September.
Novak Djokovic said it's Roger Federer and Rafael Nadal who have made him the player he is.
Novak Djokovic is now just three Grand Slam titles behind Rafael Nadal and six back from Roger Federer.
Novak Djokovic is now just three Slams behind Rafael Nadal and six back from the record 20 held by Roger Federer.
US Open 2018 Final Highlights, Juan Martin Del Potro vs Novak Djokovic Tennis Match: Novak Djokovic clinched victory in straight sets.
Juan Martin Del Potro reached the final after defending champion and world number one Rafael Nadal quit his last-four clash
Novak Djokovic boasts a 14-4 lead over Juan Martin del Potro in a 10-year rivalry, including two wins without dropping a set at the US Open in 2007 and 2012.
Third seed Juan Martin del Potro, the champion in 2009, was 7-6 (7/3), 6-2 ahead when Rafael Nadal retired.
John Isner hit big numbers in his US Open quarter-final defeat to Juan Martin del Potro, including having to change his shirt 11 times in New York's crushing heat and humidity.
Andy Murray lost 5-7, 6-2, 4-6, 4-6 to Spain's Fernando Verdasco in US Open second round.
There are no points for grace in tennis, just as there is none for sportsmanship.
The two-time Wimbledon champion will face Djokovic, a three-time winner at the All England Club, for a place in Sunday's final. Djokovic leads their epic head-to-head rivalry 26-25.
Juan Martin del Potro says he is proud to be playing in the same era as Rafael Nadal, but that burning admiration won't prevent him from trying to derail the Spaniard's Wimbledon campaign.