Osaka battled on gamely before retiring while trailing Kaia Kanepi 7-6(4), 3-0.
Three-time US Open champion Djokovic, who comes into the tournament having won his fifth Wimbledon title, is followed in the seedings by 2017 winner Rafael Nadal.
Nadal produced a rich vein of form as he thrashed Greek teenager Stefanos Tsitsipas in straight sets to clinch his fourth Canadian Masters 1000 title at the Rogers Cup
Kei Nishikori swept aside Rafael Nadal in the quarter-finals of the Rogers Cup.
