Roger Federer pulled out of the Rogers Cup on Friday without an explanation.
The tournament starts Monday in Montreal.
Federer, who is ranked fifth in the most recent ATP rankings, has had a rough couple of months, starting when he lost to 116th-ranked Sergiy Stakhovsky in the second round at Wimbledon. That loss ended Federer's streak of reaching the quarterfinals at 36 consecutive major tournaments. Then he lost to a 114th-ranked and a 55th-ranked player in consecutive tournaments.
Federer, 31, has complained about a bad back recently.
"I am disappointed not to be playing in Montreal next week," Federer said in a statement. "It is a great tournament with amazing fans. I look forward to competing there in the future."The year's last Grand Slam tournament, the U.S. Open, begins Aug. 26.