The 33-year-old former US Open champion was overcome with emotion towards the end of Delbonis's 6-1, 6-3 victory in 1hr 23min.

The former world number three has undergone four surgeries on his knee for that injury while slipping to 757 in the world rankings.

Novak Djokovic appears doomed never to be held in the same saintly esteem as Roger Federer or Rafael Nadal, the undisputed 'people's champions' of tennis.

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.