U.S. captain Jim Courier says Sock was hurt last week while playing in the doubles semifinals at the U.S. Open with Mike Bryan. Harrison was already on standby for the Davis Cup.
Courier says "it's a factor in Davis Cup all the time, having to be prepared for these last-minute changes."
Sock is ranked No. 17 in singles and No. 2 in doubles.
The winner of the best-of-five series will meet either France or Spain in the Davis Cup final.