Maria Sharapova's US Open preparations took a major blow with the Russian world number two forced to withdraw from next week's WTA event in Toronto.

The 28-year-old Russian withdrew due to a right leg strain, tournament organisers said on Friday.

Sharapova has not played since she lost to world number one Serena Williams in the Wimbledon semi-finals on July 9 and played just 10 tournament so far this year.

The Rogers Cup in Toronto is a key stop on the North American hardcourt circuit before the year's final grand slam at Flushing Meadows in New York from Aug 31-Sept 13.