Roger Federer says he made mistakes by playing when injured, though he hopes for a strong finish to his most difficult season.
Ahead of his hometown Swiss Indoors tournament, Federer cited matches he played with back problems at Indian Wells in March, and Hamburg and Gstaad in July.
Federer, ranked No. 7, says: "With hindsight, I shouldn't have done. They were errors."
The 32-year-old Swiss says he's "physically and mentally" prepared for a possible 15 matches in three season-ending weeks at Basel, the Paris Masters and ATP World Tour Finals in London.
Still, Federer says when asked if he could repeat his title hat trick of 2011 that "I don't know if my level of play is strong enough to do it now."
Federer plays Monday against 60th-ranked Frenchman Adrian Mannarino.