among the worst pieces of news yet, this year.
not 100%, but horrible nevertheless
This is a big blow for me," said Federer, winner of three Grand Slam titles this season.
"I hope I can play in Houston but I don't know. I know it will be very tight, very difficult and I don't want to take too much risk."
Federer was due to play in the Swiss Indoors in his hometown Basel but suffered the injury during practice on Monday.
"It's a huge, huge disappointment for me, obviously," said Federer. "I've been to this tournament many, many times both as a player and a ballboy.
"I really thought I had a big chance to win this year."
The 23-year-old has been told by doctors to rest for 10 to 14 days, meaning he is almost certain to miss next week's Paris Masters.