Roger Federer's faltering tennis season has suffered another twist when the former world No.1 pulled out of next week's Montreal Masters, a key warm-up for the US Open.
Federer, who is currently at five in the world, his lowest ranking for a decade, gave no reason for his withdrawal.
The 17-time grand slam title winner was beaten in the first round at Gstaad last week by Germany's Daniel Brands and admitted he had been suffering from back pain.
The Swiss great's defeat to Brands was his third of the summer against an opponent ranked outside the top 50.
He was beaten in the Wimbledon second round by Sergiy Stakhovsky and then lost in the Hamburg semi-finals to Argentine qualifier Federico Delbonis.
Federer has won only the Halle title this season and his defeat to Ukraine's world number 116 Stakhovsky in the second round at Wimbledon brought to an end a run of 36 consecutive Grand Slam quarter-final appearances.
